SQL Server DBA TSQL Interview Questions3
by Which TCP/IP Port does the SQL Server run on? How can it be Changed?
SQL Server runs on port 1433. It can be changed from the Network Utility TCP/IP properties –> Port number, both on client and the server.
What is the Difference between Clustered and a Non-clustered Index?
A clustered index is a special type of index that reorders the way records in the table are physically stored. Therefore, the table can have only one clustered index. The leaf nodes of a clustered index contain the data pages.
A non-clustered index is a special type of index in which the logical order of the index does not match the physical stored order of the rows on disk. The leaf node of a non-clustered index does not consist of the data pages. Instead, the leaf nodes contain index rows.
What are the Different Index Configurations a Table can have?
SQL Server runs on port 1433. It can be changed from the Network Utility TCP/IP properties –> Port number, both on client and the server.
What is the Difference between Clustered and a Non-clustered Index?
A clustered index is a special type of index that reorders the way records in the table are physically stored. Therefore, the table can have only one clustered index. The leaf nodes of a clustered index contain the data pages.
A non-clustered index is a special type of index in which the logical order of the index does not match the physical stored order of the rows on disk. The leaf node of a non-clustered index does not consist of the data pages. Instead, the leaf nodes contain index rows.
What are the Different Index Configurations a Table can have?
A table can have one of the following indexes configurations:
- No indexes
- A clustered index
- A clustered index and many non-clustered indexes
- A non-clustered index
- Many non-clustered indexes
What are the Different Types of Collation Sensitivity?
Case sensitivity – A and a, B and b, etc.
Accent sensitivity – a and á, o and ó, etc.
Kana Sensitivity – When Japanese Kana characters Hiragana and Katakana are treated differently, it is called Kana sensitive.
Width sensitivity – When a single-byte character (half-width) and the same character represented as a double-byte character (full-width) are treated differently, it is width sensitive.
What is OLTP (Online Transaction Processing)?
A table can have one of the following indexes configurations:
- No indexes
- A clustered index
- A clustered index and many non-clustered indexes
- A non-clustered index
- Many non-clustered indexes
What are the Different Types of Collation Sensitivity?
Case sensitivity – A and a, B and b, etc.
Accent sensitivity – a and á, o and ó, etc.
Kana Sensitivity – When Japanese Kana characters Hiragana and Katakana are treated differently, it is called Kana sensitive.
Width sensitivity – When a single-byte character (half-width) and the same character represented as a double-byte character (full-width) are treated differently, it is width sensitive.
What is OLTP (Online Transaction Processing)?
In OLTP –(online transaction processing) systems, relational database design uses the discipline of data modeling and generally follows the Codd rules of data normalization in order to ensure absolute data integrity. Using these rules, complex information is broken down into its most simple structures (a table) where all of the individual atomic level elements relate to each other and satisfy the normalization rules.
What’s the Difference between a Primary Key and a Unique Key?
In OLTP –(online transaction processing) systems, relational database design uses the discipline of data modeling and generally follows the Codd rules of data normalization in order to ensure absolute data integrity. Using these rules, complex information is broken down into its most simple structures (a table) where all of the individual atomic level elements relate to each other and satisfy the normalization rules.
What’s the Difference between a Primary Key and a Unique Key?
Both primary key and unique key enforces uniqueness of the column on which they are defined. But by default, the primary key creates a clustered index on the column, whereas unique key creates a non-clustered index by default. Another major difference is that primary key doesn’t allow NULLs, but unique key allows one NULL only.
What is the Difference between DELETE and TRUNCATE Commands?
Both primary key and unique key enforces uniqueness of the column on which they are defined. But by default, the primary key creates a clustered index on the column, whereas unique key creates a non-clustered index by default. Another major difference is that primary key doesn’t allow NULLs, but unique key allows one NULL only.
What is the Difference between DELETE and TRUNCATE Commands?
The delete command removes the rows from a table on the basis of the condition that we provide with a WHERE clause. Truncate will actually remove all the rows from a table, and there will be no data in the table after we run the truncate command.
The delete command removes the rows from a table on the basis of the condition that we provide with a WHERE clause. Truncate will actually remove all the rows from a table, and there will be no data in the table after we run the truncate command.
TRUNCATE
- TRUNCATE is faster and uses fewer system and transaction log resources than DELETE. (Read all the points below)
- TRUNCATE removes the data by deallocating the data pages used to store the table’s data, and only the page deallocations are recorded in the transaction log.
- TRUNCATE removes all the rows from a table, but the table structure, its columns, constraints, indexes and so on remains. The counter used by an identity for new rows is reset to the seed for the column.
- You cannot use TRUNCATE TABLE on a table referenced by a FOREIGN KEY constraint.
- Using T-SQL – TRUNCATE cannot be rolled back unless it is used in TRANSACTION. OR TRUNCATE can be rolled back when used with BEGIN … END TRANSACTION using T-SQL.
- TRUNCATE is a DDL Command.
- TRUNCATE resets the identity of the table.
DELETE
- DELETE removes rows one at a time and records an entry in the transaction log for each deleted row.
- DELETE does not reset Identity property of the table.
- DELETE can be used with or without a WHERE clause
- DELETE activates Triggers if defined on the table.
- DELETE can be rolled back.
- DELETE is DML Command.
- DELETE does not reset the identity of the table.
What are Different Types of Locks?
- Shared Locks: Used for operations that do not change or update data (read-only operations), such as a SELECT statement.
- Update Locks: Used on resources that can be updated. It prevents a common form of deadlock that occurs when multiple sessions are reading, locking, and potentially updating resources later.
- Exclusive Locks: Used for data-modification operations, such as INSERT, UPDATE, or DELETE. It ensures that multiple updates cannot be made to the same resource at the same time.
- Intent Locks: Used to establish a lock hierarchy. The types of intent locks are as follows: intent shared (IS), intent exclusive (IX), and shared with intent exclusive (SIX).
- Schema Locks: Used when an operation dependent on the schema of a table is executing. The types of schema locks are schema modification (Sch-M) and schema stability (Sch-S).
- Bulk Update Locks: Used when bulk-copying data into a table and the TABLOCK hint is specified.
What are Pessimistic Lock and Optimistic Lock?
Optimistic Locking is a strategy where you read a record, take note of a version number and check that the version hasn’t changed before you write the record back. If the record is dirty (i.e. different version to yours), then you abort the transaction and the user can re-start it.
Pessimistic Locking is when you lock the record for your exclusive use until you have finished with it. It has much better integrity than optimistic locking but requires you to be careful with your application design to avoid Deadlocks.
When is the use of UPDATE_STATISTICS command?
- TRUNCATE is faster and uses fewer system and transaction log resources than DELETE. (Read all the points below)
- TRUNCATE removes the data by deallocating the data pages used to store the table’s data, and only the page deallocations are recorded in the transaction log.
- TRUNCATE removes all the rows from a table, but the table structure, its columns, constraints, indexes and so on remains. The counter used by an identity for new rows is reset to the seed for the column.
- You cannot use TRUNCATE TABLE on a table referenced by a FOREIGN KEY constraint.
- Using T-SQL – TRUNCATE cannot be rolled back unless it is used in TRANSACTION. OR TRUNCATE can be rolled back when used with BEGIN … END TRANSACTION using T-SQL.
- TRUNCATE is a DDL Command.
- TRUNCATE resets the identity of the table.
DELETE
- DELETE removes rows one at a time and records an entry in the transaction log for each deleted row.
- DELETE does not reset Identity property of the table.
- DELETE can be used with or without a WHERE clause
- DELETE activates Triggers if defined on the table.
- DELETE can be rolled back.
- DELETE is DML Command.
- DELETE does not reset the identity of the table.
What are Different Types of Locks?
- Shared Locks: Used for operations that do not change or update data (read-only operations), such as a SELECT statement.
- Update Locks: Used on resources that can be updated. It prevents a common form of deadlock that occurs when multiple sessions are reading, locking, and potentially updating resources later.
- Exclusive Locks: Used for data-modification operations, such as INSERT, UPDATE, or DELETE. It ensures that multiple updates cannot be made to the same resource at the same time.
- Intent Locks: Used to establish a lock hierarchy. The types of intent locks are as follows: intent shared (IS), intent exclusive (IX), and shared with intent exclusive (SIX).
- Schema Locks: Used when an operation dependent on the schema of a table is executing. The types of schema locks are schema modification (Sch-M) and schema stability (Sch-S).
- Bulk Update Locks: Used when bulk-copying data into a table and the TABLOCK hint is specified.
What are Pessimistic Lock and Optimistic Lock?
Optimistic Locking is a strategy where you read a record, take note of a version number and check that the version hasn’t changed before you write the record back. If the record is dirty (i.e. different version to yours), then you abort the transaction and the user can re-start it.
Pessimistic Locking is when you lock the record for your exclusive use until you have finished with it. It has much better integrity than optimistic locking but requires you to be careful with your application design to avoid Deadlocks.
When is the use of UPDATE_STATISTICS command?
This command is basically used when a large amount of data is processed. If a large amount of deletions, modifications or Bulk Copy into the tables has occurred, it has to update the indexes to take these changes into account. UPDATE_STATISTICS updates the indexes on these tables accordingly.
What is the Difference between a HAVING clause and a WHERE clause?
They specify a search condition for a group or an aggregate. But the difference is that HAVING can be used only with the SELECT statement. HAVING is typically used in a GROUP BY clause. When GROUP BY is not used, HAVING behaves like a WHERE clause. Having Clause is basically used only with the GROUP BY function in a query, whereas WHERE Clause is applied to each row before they are part of the GROUP BY function in a query.
What is Connection Pooling and why it is Used?
This command is basically used when a large amount of data is processed. If a large amount of deletions, modifications or Bulk Copy into the tables has occurred, it has to update the indexes to take these changes into account. UPDATE_STATISTICS updates the indexes on these tables accordingly.
What is the Difference between a HAVING clause and a WHERE clause?
They specify a search condition for a group or an aggregate. But the difference is that HAVING can be used only with the SELECT statement. HAVING is typically used in a GROUP BY clause. When GROUP BY is not used, HAVING behaves like a WHERE clause. Having Clause is basically used only with the GROUP BY function in a query, whereas WHERE Clause is applied to each row before they are part of the GROUP BY function in a query.
What is Connection Pooling and why it is Used?
To minimize the cost of opening and closing connections, ADO.NET uses an optimization technique called connection pooling.
The pooler maintains ownership of the physical connection. It manages connections by keeping alive a set of active connections for each given connection configuration. Whenever a user calls Open on a connection, the pooler looks for an available connection in the pool. If a pooled connection is available, it returns it to the caller instead of opening a new connection. When the application calls Close on the connection, the pooler returns it to the pooled set of active connections instead of closing it. Once the connection is returned to the pool, it is ready to be reused on the next Open call.
What are the Properties and Different Types of Sub-Queries?
To minimize the cost of opening and closing connections, ADO.NET uses an optimization technique called connection pooling.
The pooler maintains ownership of the physical connection. It manages connections by keeping alive a set of active connections for each given connection configuration. Whenever a user calls Open on a connection, the pooler looks for an available connection in the pool. If a pooled connection is available, it returns it to the caller instead of opening a new connection. When the application calls Close on the connection, the pooler returns it to the pooled set of active connections instead of closing it. Once the connection is returned to the pool, it is ready to be reused on the next Open call.
What are the Properties and Different Types of Sub-Queries?
Properties of a Sub-Query
- A sub-query must be enclosed in the parenthesis.
- A sub-query must be put on the right hand of the comparison operator, and
- A sub-query cannot contain an ORDER BY clause, however sub-query can use ORDER BY when used with TOP clause.
- A query can contain more than one sub-query.
Types of Sub-query
- Single-row sub-query, where the sub-query returns only one row.
- Multiple-row sub-query, where the sub-query returns multiple rows, and
- Multiple column sub-query, where the sub-query returns multiple columns
What is an SQL Profiler?
SQL Profiler is a graphical tool that allows system administrators to monitor events in an instance of Microsoft SQL Server. You can capture and save data about each event to a file or SQL Server table to analyze later. For example, you can monitor a production environment to see which stored procedures are hampering performances by executing very slowly.
Use SQL Profiler to monitor only the events in which you are interested. If traces are becoming too large, you can filter them based on the information you want, so that only a subset of the event data is collected. Monitoring too many events adds overhead to the server and the monitoring process and can cause the trace file or trace table to grow very large, especially when the monitoring process takes place over a long period of time.
What are the Authentication Modes in SQL Server? How can it be Changed?
- A sub-query must be enclosed in the parenthesis.
- A sub-query must be put on the right hand of the comparison operator, and
- A sub-query cannot contain an ORDER BY clause, however sub-query can use ORDER BY when used with TOP clause.
- A query can contain more than one sub-query.
Types of Sub-query
- Single-row sub-query, where the sub-query returns only one row.
- Multiple-row sub-query, where the sub-query returns multiple rows, and
- Multiple column sub-query, where the sub-query returns multiple columns
What is an SQL Profiler?
SQL Profiler is a graphical tool that allows system administrators to monitor events in an instance of Microsoft SQL Server. You can capture and save data about each event to a file or SQL Server table to analyze later. For example, you can monitor a production environment to see which stored procedures are hampering performances by executing very slowly.
Use SQL Profiler to monitor only the events in which you are interested. If traces are becoming too large, you can filter them based on the information you want, so that only a subset of the event data is collected. Monitoring too many events adds overhead to the server and the monitoring process and can cause the trace file or trace table to grow very large, especially when the monitoring process takes place over a long period of time.
What are the Authentication Modes in SQL Server? How can it be Changed?
There are two authentication modes in SQL Server.
- Windows Mode
- Mixed Mode – SQL and Windows
To change authentication mode in SQL Server, go to Start -> Programs- > Microsoft SQL Server and click SQL Enterprise Manager to run SQL Enterprise Manager from the Microsoft SQL Server program group. Select the server; then from the Tools menu, select SQL Server Configuration Properties and choose the Security page.
Which Command using Query Analyzer will give you the Version of SQL Server and Operating System?
There are two authentication modes in SQL Server.
- Windows Mode
- Mixed Mode – SQL and Windows
To change authentication mode in SQL Server, go to Start -> Programs- > Microsoft SQL Server and click SQL Enterprise Manager to run SQL Enterprise Manager from the Microsoft SQL Server program group. Select the server; then from the Tools menu, select SQL Server Configuration Properties and choose the Security page.
Which Command using Query Analyzer will give you the Version of SQL Server and Operating System?
SELECT SERVERPROPERTY('Edition') AS Edition, SERVERPROPERTY('ProductLevel') AS ProductLevel, |
What is an SQL Server Agent?
The SQL Server agent plays an important role in the day-to-day tasks of a database administrator (DBA). It is often overlooked as one of the main tools for SQL Server management. Its purpose is to ease the implementation of tasks for the DBA, with its full-function scheduling engine, which allows you to schedule your own jobs and scripts.
Can a Stored Procedure call itself or a Recursive Stored Procedure? How many levels of SP nesting is possible?
What is an SQL Server Agent?
The SQL Server agent plays an important role in the day-to-day tasks of a database administrator (DBA). It is often overlooked as one of the main tools for SQL Server management. Its purpose is to ease the implementation of tasks for the DBA, with its full-function scheduling engine, which allows you to schedule your own jobs and scripts.
Can a Stored Procedure call itself or a Recursive Stored Procedure? How many levels of SP nesting is possible?
Yes. As T-SQL supports recursion, you can write stored procedures that call themselves. Recursion can be defined as a method of problem solving wherein the solution is arrived at by repetitively applying it to subsets of the problem. A common application of recursive logic is to perform numeric computations that lend themselves to repetitive evaluation by the same processing steps. Stored procedures are nested when one stored procedure calls another or executes managed code by referencing a CLR routine, type, or aggregate. You can nest stored procedures up to 32 levels. Any reference to managed code from a Transact-SQL stored procedure counts as one level against the 32-level nesting limit. Methods invoked from within managed code do not count against this limit.
What is Log Shipping?
Yes. As T-SQL supports recursion, you can write stored procedures that call themselves. Recursion can be defined as a method of problem solving wherein the solution is arrived at by repetitively applying it to subsets of the problem. A common application of recursive logic is to perform numeric computations that lend themselves to repetitive evaluation by the same processing steps. Stored procedures are nested when one stored procedure calls another or executes managed code by referencing a CLR routine, type, or aggregate. You can nest stored procedures up to 32 levels. Any reference to managed code from a Transact-SQL stored procedure counts as one level against the 32-level nesting limit. Methods invoked from within managed code do not count against this limit.
What is Log Shipping?
Log shipping is the process of automating the backup of database and transaction log files on a production SQL server and then restoring them onto a standby server. All Editions (except Express Edition) supports log shipping. In log shipping, the transactional log file from one server is automatically updated into the backup database on the other server. If one server fails, the other server will have the same db and can be used this as the Disaster Recovery plan. The key feature of log shipping is that it will automatically backup transaction logs throughout the day and automatically restore them on the standby server at defined intervals.
Name 3 ways to get an Accurate Count of the Number of Records in a Table?
Log shipping is the process of automating the backup of database and transaction log files on a production SQL server and then restoring them onto a standby server. All Editions (except Express Edition) supports log shipping. In log shipping, the transactional log file from one server is automatically updated into the backup database on the other server. If one server fails, the other server will have the same db and can be used this as the Disaster Recovery plan. The key feature of log shipping is that it will automatically backup transaction logs throughout the day and automatically restore them on the standby server at defined intervals.
Name 3 ways to get an Accurate Count of the Number of Records in a Table?
SELECT * FROM table1 SELECT COUNT(*) FROM table1 SELECT rows FROM sysindexes WHERE id = OBJECT_ID(table1) AND indid < 2 |
When SET QUOTED_IDENTIFIER is ON, identifiers can be delimited by double quotation marks, and literals must be delimited by single quotation marks. When SET QUOTED_IDENTIFIER is OFF, identifiers cannot be quoted and must follow all T-SQL rules for identifiers.
What is the Difference between a Local and a Global Temporary Table?
A local temporary table exists only for the duration of a connection, or if defined inside a compound statement, for the duration of the compound statement.
A global temporary table remains in the database accessible across the connections. Once the connection where original global table is declared dropped this becomes unavailable.
What is the STUFF Function and How Does it Differ from the REPLACE Function?
When SET QUOTED_IDENTIFIER is ON, identifiers can be delimited by double quotation marks, and literals must be delimited by single quotation marks. When SET QUOTED_IDENTIFIER is OFF, identifiers cannot be quoted and must follow all T-SQL rules for identifiers.
What is the Difference between a Local and a Global Temporary Table?
A local temporary table exists only for the duration of a connection, or if defined inside a compound statement, for the duration of the compound statement.
A global temporary table remains in the database accessible across the connections. Once the connection where original global table is declared dropped this becomes unavailable.
What is the STUFF Function and How Does it Differ from the REPLACE Function?
STUFF function is used to overwrite existing characters using this syntax: STUFF (string_expression, start, length, replacement_characters), where string_expression is the string that will have characters substituted, start is the starting position, length is the number of characters in the string that are substituted, and replacement_characters are the new characters interjected into the string. REPLACE function is used to replace existing characters of all occurrences. Using the syntax REPLACE (string_expression, search_string, replacement_string), every incidence of search_string found in the string_expression will be replaced with replacement_string.
What is PRIMARY KEY?
STUFF function is used to overwrite existing characters using this syntax: STUFF (string_expression, start, length, replacement_characters), where string_expression is the string that will have characters substituted, start is the starting position, length is the number of characters in the string that are substituted, and replacement_characters are the new characters interjected into the string. REPLACE function is used to replace existing characters of all occurrences. Using the syntax REPLACE (string_expression, search_string, replacement_string), every incidence of search_string found in the string_expression will be replaced with replacement_string.
What is PRIMARY KEY?
A PRIMARY KEY constraint is a unique identifier for a row within a database table. Every table should have a primary key constraint to uniquely identify each row, and only one primary key constraint can be created for each table. The primary key constraints are used to enforce entity integrity.
What is UNIQUE KEY Constraint?
A PRIMARY KEY constraint is a unique identifier for a row within a database table. Every table should have a primary key constraint to uniquely identify each row, and only one primary key constraint can be created for each table. The primary key constraints are used to enforce entity integrity.
What is UNIQUE KEY Constraint?
A UNIQUE constraint enforces the uniqueness of the values in a set of columns; so no duplicate values are entered. The unique key constraints are used to enforce entity integrity as the primary key constraints.
What is FOREIGN KEY?
A UNIQUE constraint enforces the uniqueness of the values in a set of columns; so no duplicate values are entered. The unique key constraints are used to enforce entity integrity as the primary key constraints.
What is FOREIGN KEY?
A FOREIGN KEY constraint prevents any actions that would destroy links between tables with the corresponding data values. A foreign key in one table points to a primary key in another table. Foreign keys prevent actions that would leave rows with foreign key values when there are no primary keys with that value. The foreign key constraints are used to enforce referential integrity.
What is CHECK Constraint?
A CHECK constraint is used to limit the values that can be placed in a column. The check constraints are used to enforce domain integrity.
What is NOT NULL Constraint?
A FOREIGN KEY constraint prevents any actions that would destroy links between tables with the corresponding data values. A foreign key in one table points to a primary key in another table. Foreign keys prevent actions that would leave rows with foreign key values when there are no primary keys with that value. The foreign key constraints are used to enforce referential integrity.
What is CHECK Constraint?
A CHECK constraint is used to limit the values that can be placed in a column. The check constraints are used to enforce domain integrity.
What is NOT NULL Constraint?
A NOT NULL constraint enforces that the column will not accept null values. The not null constraints are used to enforce domain integrity, as the check constraints.
What is the difference between UNION and UNION ALL?
A NOT NULL constraint enforces that the column will not accept null values. The not null constraints are used to enforce domain integrity, as the check constraints.
What is the difference between UNION and UNION ALL?
UNION
The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type. With UNION, only distinct values are selected.
UNION ALL
The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.The difference between UNION and UNION ALL is that UNION ALL will not eliminate duplicate rows, instead it just pulls all rows from all the tables fitting your query specifics and combines them into a table.
What is B-Tree?
UNION
The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type. With UNION, only distinct values are selected.
UNION ALL
The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.The difference between UNION and UNION ALL is that UNION ALL will not eliminate duplicate rows, instead it just pulls all rows from all the tables fitting your query specifics and combines them into a table.
What is B-Tree?
The database server uses a B-tree structure to organize index information. B-Tree generally has following types of index pages or nodes:
- Root node: A root node contains node pointers to only one branch node.
- Branch nodes: A branch node contains pointers to leaf nodes or other branch nodes, which can be two or more.
- Leaf nodes: A leaf node contains index items and horizontal pointers to other leaf nodes, which can be many.
How to get @@ERROR and @@ROWCOUNT at the Same Time?
If @@Rowcount is checked after Error checking statement, then it will have 0 as the value of @@Recordcount as it would have been reset. And if @@Recordcount is checked before the error-checking statement, then @@Error would get reset. To get @@error and @@rowcount at the same time, include both in same statement and store them in a local variable. SELECT @RC = @@ROWCOUNT, @ER = @@ERROR
What is a Scheduled Job or What is a Scheduled Task?
The database server uses a B-tree structure to organize index information. B-Tree generally has following types of index pages or nodes:
- Root node: A root node contains node pointers to only one branch node.
- Branch nodes: A branch node contains pointers to leaf nodes or other branch nodes, which can be two or more.
- Leaf nodes: A leaf node contains index items and horizontal pointers to other leaf nodes, which can be many.
How to get @@ERROR and @@ROWCOUNT at the Same Time?
If @@Rowcount is checked after Error checking statement, then it will have 0 as the value of @@Recordcount as it would have been reset. And if @@Recordcount is checked before the error-checking statement, then @@Error would get reset. To get @@error and @@rowcount at the same time, include both in same statement and store them in a local variable. SELECT @RC = @@ROWCOUNT, @ER = @@ERROR
What is a Scheduled Job or What is a Scheduled Task?
Scheduled tasks let user automate processes that run on regular or predictable cycles. User can schedule administrative tasks, such as cube processing, to run during times of slow business activity. User can also determine the order in which tasks run by creating job steps within a SQL Server Agent job, e.g. back up database and update statistics of the tables. Job steps give user control over flow of execution. If one job fails, then the user can configure SQL Server Agent to continue to run the remaining tasks or to stop execution.
What are the Advantages of Using Stored Procedures?
Scheduled tasks let user automate processes that run on regular or predictable cycles. User can schedule administrative tasks, such as cube processing, to run during times of slow business activity. User can also determine the order in which tasks run by creating job steps within a SQL Server Agent job, e.g. back up database and update statistics of the tables. Job steps give user control over flow of execution. If one job fails, then the user can configure SQL Server Agent to continue to run the remaining tasks or to stop execution.
What are the Advantages of Using Stored Procedures?
- Stored procedure can reduced network traffic and latency, boosting application performance.
- Stored procedure execution plans can be reused; they staying cached in SQL Server’s memory, reducing server overhead.
- Stored procedures help promote code reuse.
- Stored procedures can encapsulate logic. You can change stored procedure code without affecting clients.
- Stored procedures provide better security to your data.
What is a Table Called, if it has neither Cluster nor Non-cluster Index? What is it Used for?
Unindexed table or Heap. Microsoft Press Books and Book on Line (BOL) refers it as Heap. A heap is a table that does not have a clustered index and therefore, the pages are not linked by pointers. The IAM pages are the only structures that link the pages in a table together. Unindexed tables are good for fast storing of data. Many times, it is better to drop all the indexes from table and then do bulk of INSERTs and restore those indexes after that.
Can SQL Servers Linked to other Servers like Oracle?
SQL Server can be linked to any server provided it has OLE-DB provider from Microsoft to allow a link, e.g. Oracle has an OLE-DB provider that Microsoft provides to add it as a linked server to the SQL Server group.
What is BCP? When is it Used?
- Stored procedure can reduced network traffic and latency, boosting application performance.
- Stored procedure execution plans can be reused; they staying cached in SQL Server’s memory, reducing server overhead.
- Stored procedures help promote code reuse.
- Stored procedures can encapsulate logic. You can change stored procedure code without affecting clients.
- Stored procedures provide better security to your data.
What is a Table Called, if it has neither Cluster nor Non-cluster Index? What is it Used for?
Unindexed table or Heap. Microsoft Press Books and Book on Line (BOL) refers it as Heap. A heap is a table that does not have a clustered index and therefore, the pages are not linked by pointers. The IAM pages are the only structures that link the pages in a table together. Unindexed tables are good for fast storing of data. Many times, it is better to drop all the indexes from table and then do bulk of INSERTs and restore those indexes after that.
Can SQL Servers Linked to other Servers like Oracle?
SQL Server can be linked to any server provided it has OLE-DB provider from Microsoft to allow a link, e.g. Oracle has an OLE-DB provider that Microsoft provides to add it as a linked server to the SQL Server group.
What is BCP? When is it Used?
BCP or BulkCopy is a tool used to copy huge amounts of data from tables and views. BCP does not copy the complete structures from source to destination. BULK INSERT command helps to import a data file into a database table or view in a user-specified format.
What Command do we Use to Rename a db, a Table and a Column?
BCP or BulkCopy is a tool used to copy huge amounts of data from tables and views. BCP does not copy the complete structures from source to destination. BULK INSERT command helps to import a data file into a database table or view in a user-specified format.
What Command do we Use to Rename a db, a Table and a Column?
To Rename db
sp_renamedb ‘oldname’ , ‘newname |
USE MASTER; GO |
To Rename Table
We can change the table name using sp_rename as follows:
sp_rename 'oldTableName' 'newTableName' |
To rename Column
The script for renaming any column is as follows:
sp_rename 'TableName.[OldcolumnName]', 'NewColumnName', 'Column' |
e.g
sp_RENAME 'Table_First.Name', 'NameChange' , 'COLUMN' GO |
What are sp_configure Commands and SET Commands?
Use sp_configure to display or change server-level settings. To change the database-level settings, use ALTER DATABASE. To change settings that affect only the current user session, use the SET statement.e.g.
sp_CONFIGURE 'show advanced', 0 |
You can run the following command and check the advanced global configuration settings.
sp_CONFIGURE 'show advanced', 1 |
How to Implement One-to-One, One-to-Many and Many-to-Many Relationships while Designing Tables?
One-to-One relationship can be implemented as a single table and rarely as two tables with primary and foreign key relationships. One-to-Many relationships are implemented by splitting the data into two tables with primary key and foreign key relationships.
Many-to-Many relationships are implemented using a junction table with the keys from both the tables forming the composite primary key of the junction table.
What is Difference between Commit and Rollback when Used in Transactions?
The usual structure of the TRANSACTION is as follows:
| BEGIN TRANSACTION operations COMMIT TRANSACTION or ROLLBACK TRANSACTION |
When Commit is executed, every statement between BEGIN and COMMIT becomes persistent to database. When Rollback is executed, every statement between BEGIN and ROLLBACK are reverted to the state when BEGIN was executed.
What is an Execution Plan? When would you Use it? How would you View the Execution Plan?
An execution plan is basically a road map that graphically or textually shows the data retrieval methods chosen by the SQL Server query optimizer for a stored procedure or ad-hoc query, and it is a very useful tool for a developer to understand the performance characteristics of a query or stored procedure since the plan is the one that SQL Server will place in its cache and use to execute the stored procedure or query. Within the Query Analyzer, there is an option called “Show Execution Plan†(in the Query drop-down menu). If this option is turned on, it will display query execution plan in a separate window when the query is ran again.
What is Difference between Table Aliases and Column Aliases? Do they Affect Performance?
Usually, when the name of the table or column is very long or complicated to write, aliases are used to refer them.e.g
| SELECT verylongcolumnname col1 FROM verylongtablename tab1 |
In the above example, col1 and tab1 are the column alias and table alias, respectively. They do not affect the performance at all.
What is the difference between CHAR and VARCHAR Datatypes?
VARCHARS are variable length strings with a specified maximum length. If a string is less than the maximum length, then it is stored verbatim without any extra characters, e.g. names and emails. CHARS are fixed-length strings with a specified set length. If a string is less than the set length, then it is padded with extra characters, e.g. phone number and zip codes. For instance, for a column which is declared as VARCHAR(30) and populated with the word ‘SQL Server,’ only 10 bytes will be stored in it. However, if we have declared the column as CHAR(30) and populated with the word ‘SQL Server,’ it will still occupy 30 bytes in database.
What is the Difference between VARCHAR and VARCHAR(MAX) Datatypes?
VARCHAR stores variable-length character data whose range varies up to 8000 bytes; varchar(MAX) stores variable-length character data whose range may vary beyond 8000 bytes and till 2 GB. TEXT datatype is going to be deprecated in future versions, and the usage of VARCHAR(MAX) is strongly recommended instead of TEXT datatypes.
What is the Difference between VARCHAR and NVARCHAR datatypes?
In principle, they are the same and are handled in the same way by your application. The only difference is that NVARCHAR can handle unicode characters, allowing you to use multiple languages in the database (Arabian, Chinese, etc.). NVARCHAR takes twice as much space when compared to VARCHAR. Use NVARCHAR only if you are using foreign languages.
Which are the Important Points to Note when Multilanguage Data is Stored in a Table?
There are two things to keep in mind while storing unicode data. First, the column must be of unicode data type (nchar, nvarchar, ntext). Second, the value must be prefixed with N while insertion. For example,
INSERT INTO table (Hindi_col) values (N’hindi data’)
How to Optimize Stored Procedure Optimization?
There are many tips and tricks for the same. Here are few:
- Include SET NOCOUNT ON statement.
- Use schema name with object name.
- Do not use the prefix “sp_†in the stored procedure name.
- Use IF EXISTS (SELECT 1) instead of (SELECT *).
- Use the sp_executesql stored procedure instead of the EXECUTE statement.
- Try to avoid using SQL Server cursors whenever possible.
- Keep the Transaction as short as possible.
- Use TRY-Catch for error handling.
What is SQL Injection? How to Protect Against SQL Injection Attack?
SQL injection is an attack in which malicious code is inserted into strings that are later passed to an instance of SQL Server for parsing and execution. Any procedure that constructs SQL statements should be reviewed for injection vulnerabilities because SQL Server will execute all syntactically valid queries that it receives. Even parameterized data can be manipulated by a skilled and determined attacker.
Here are few methods which can be used to protect again SQL Injection attack:
- Use Type-Safe SQL Parameters
- Use Parameterized Input with Stored Procedures
- Use the Parameters Collection with Dynamic SQL
- Filtering Input parameters
- Use the escape character in LIKE clause
- Wrapping Parameters with QUOTENAME() and REPLACE()
How to Find Out the List Schema Name and Table Name for the Database?
We can use following script:
SELECT '[' + Schema_name(schema_id) + '].[' + NAME + ']' AS SchemaTable FROM sys.tables |
CHECKPOINT process writes all dirty pages for the current database to disk. Dirty pages are data pages that have been entered into the buffer cache and modified, but not yet written to disk.
How does Using a Separate Hard Drive for Several Database Objects Improves Performance Right Away?
A non-clustered index and tempdb can be created on a separate disk to improve performance.
How to Find the List of Fixed Hard Drive and Free Space on Server?
We can use the following Stored Procedure to figure out the number of fixed drives (hard drive) a system has along with free space on each of those drives.
| EXEC master.. Xp_fixeddrives |
Why can there be only one Clustered Index and not more than one?
Cluster Index physically stores data, or arranges data in one order (depends on which column(s) you have defined Clustered index and in which order).As a fact, we all know that a set of data can be only stored in only one order; that is why only one clustered index is possible.
What is Difference between Line Feed (\n) and Carriage Return (\r)?
Line Feed – LF – \n – 0x0a – 10 (decimal)
Carriage Return – CR – \r – 0x0D – 13 (decimal)
DECLARE @NewLineChar AS CHAR(2) = CHAR(13) + CHAR(10) PRINT ('SELECT FirstLine AS FL ' +@NewLineChar + 'SELECT SecondLine AS SL' ) |
Is It Possible to have Clustered Index on Separate Drive From Original Table Location?
No! It is not possible.
What is a Hint?
Hints are options and strong suggestions specified for enforcement by the SQL Server query processor on DML statements. The hints override any execution plan the query optimizer might select for a query.There are three different types of hints. Let us understand the basics of each of them separately.
Join Hint
This hint is used when more than one table is used in a query. Two or more tables can be joined using different types of joins. This hint forces the type of join algorithm that is used. Joins can be used in SELECT, UPDATE and DELETE statements.
Query Hint
This hint is used when certain kind of logic has to be applied to a whole query. Any hint used in the query is applied to the complete query as opposed to a part of it. There is no way to specify that only a certain part of a query should be used with the hint. After any query, the OPTION clause is specified to apply the logic to this query. A query always has any of the following statements: SELECT, UPDATE, DELETE, INSERT or MERGE (SQL 2K8); and this hint can be applied to all of them.
Table Hint
This hint is used when certain kind of locking mechanism of tables has to be controlled. SQL Server query optimizer always puts the appropriate kind of lock on tables, when any of the Transact SQL operations SELECT, UPDATE, DELETE, INSERT or MERGE is used. There are certain cases when the developer knows when and where to override the default behavior of the locking algorithm, and these hints are useful in those scenarios.
How to Delete Duplicate Rows?
We can delete duplicate rows using CTE and ROW_NUMBER () feature of SQL Server 2005 and SQL Server 2008.e.g.
WITH CTE (COl1,Col2, DuplicateCount) AS |
Why the Trigger Fires Multiple Times in Single Login?
It happens because multiple SQL Server services are running and also as intellisense is turned on.
What is Aggregate Functions?
Aggregate functions perform a calculation on a set of values and return a single value. Aggregate functions ignore NULL values except COUNT function. HAVING clause is used, along with GROUP BY for filtering query using aggregate values.
The following functions are aggregate functions.
AVG, MIN, CHECKSUM_AGG, SUM, COUNT, STDEV, COUNT_BIG, STDEVP, GROUPING, VAR, MAX, VARP
What is Use of @@ SPID in SQL Server?
A SPID is the returns sessions ID of the current user process. And using that session ID, we can find out that the last query was executed.
What is the Difference between Index Seek Vs. Index Scan?
An index scan means that SQL Server reads all the rows in a table, and then returns only those rows that satisfy the search criteria. When an index scan is performed, all the rows in the leaf level of the index are scanned. This essentially means that all the rows of the index are examined instead of the table directly. This is sometimes compared to a table scan, in which all the table data is read directly. However, there is usually little difference between an index scan and a table scan.
An index seek, on the other hand, means that the Query Optimizer relies entirely on the index leaf data to locate rows satisfying the query condition. An index seek will be most beneficial in cases where a small percentage of rows will be returned. An index seek will only affect the rows that satisfy a query condition and the pages that contain these qualifying rows; in terms of performance, this is highly beneficial when a table has a very large number of rows.
What is the Maximum Size per Database for SQL Server Express?
SQL Server Express supports a maximum size of 4 GB per database, which excludes all the log files. 4 GB is not a very large size; however, if the database is properly designed and the tables are properly arranged in a separate database, this limitation can be resolved to a certain extent.
How do We Know if Any Query is Retrieving a Large Amount of Data or very little data?
In one way, it is quite easy to figure this out by just looking at the result set; however, this method cannot be relied upon every time as it is difficult to reach a conclusion when there are many columns and many rows.It is easy to measure how much data is retrieved from server to client side. The SQL Server Management Studio has feature that can measure client statistics.
What is the Difference between GRANT and WITH GRANT while Giving Permissions to the User?
In case of only GRANT, the username cannot grant the same permission to other users. On the other hand, with the option WITH GRANT, the username will be able to give the permission after receiving requests from other users.
How to Create Primary Key with Specific Name while Creating a Table?
CREATE TABLE [dbo].[TestTable]( [ID] [int] IDENTITY(1,1)NOTNULL, |
What is T-SQL Script to Take Database Offline – Take Database Online?
-- Take the Database Offline ALTER DATABASE [myDB] SET OFFLINE WITH |
--Disable Index ALTER INDEX [IndexName] ON TableName DISABLE |
Can we Insert Data if Clustered Index is Disabled?
No, we cannot insert data if Clustered Index is disabled because Clustered Indexes are in fact original tables which are physically ordered according to one or more keys (Columns).
How to Recompile Stored Procedure at Run Time?
We can Recompile Stored Procedure in two ways.
Option 1:
CREATE PROCEDURE dbo.PersonAge(@MinAge INT, @MaxAge INT) WITH RECOMPILE |
Option 2:
EXEC dbo.PersonAge65, 70 WITHRECOMPILE |
We can use RECOMPILE hint with a query and recompile only that particular query. However, if the parameters are used in many statements in the stored procedure and we want to recompile all the statements, then instead of using the RECOMPILE option with all the queries, we have one better option that uses WITH RECOMPILE during stored procedure creation or execution.This method is not recommended for large stored procedures because the recompilation of so many statements may outweigh the benefit of a better execution plan.
Is there any Performance Difference between IF EXISTS (Select null from table) and IF EXISTS (Select 1 from table)?
There is no performance difference between IF EXISTS (Select null from table) and IF EXISTS (Select 1 from table).
What is Difference in Performance between INSERT TOP (N) INTO Table and Using Top with INSERT?
INSERT TOP (N) INTO Table is faster than Using Top with INSERT but when we use INSERT TOP (N) INTO Table, the ORDER BY clause is totally ignored.
Does the Order of Columns in UPDATE statements Matter?
No, the order of columns in UPDATE statement does not matter for results updated.Both the below options produce the same results.
Option 1:
UPDATE TableName SET Col1 ='Value', Col2 ='Value2' |
Option 2:
UPDATE TableName SET Col2 ='Value2', Col1 ='Value' |
Statistics Says It All
16
Years of Experience
3000
Gratified Students
100
Training Batches
9600
Training Hours
Subscribe
Please subscribe our technical blog to get recent updates.