SQL Server Interview Questions

SQL Server is a relational database management system (RDBMS) developed by Microsoft. It is primarily designed and developed to compete with MySQL and Oracle database. So as a SQL Server professionals you might have faced these interview questions or going to face in your next job interview. So you can try these and prepare well.

Q.1 How can you enable and use Snapshot Isolation in SQL Server?

Snapshot Isolation can be enabled at the database level using the ALTER DATABASE command with the ALLOW_SNAPSHOT_ISOLATION option set to ON. After enabling it, transactions can use the SET TRANSACTION ISOLATION LEVEL SNAPSHOT statement to utilize Snapshot Isolation.

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Q.2 What are the potential performance trade-offs when using Row Versioning?

Row Versioning increases the storage overhead due to the need to store versions of modified rows. Additionally, it might lead to increased tempdb usage, as row versions are stored there. However, the performance benefits of reduced blocking and improved concurrency often outweigh these overheads.

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Q.3 How does SQL Server manage row versions and when are they removed?

SQL Server uses the version store in tempdb to store row versions. Versions are removed when they are no longer needed for ongoing transactions, such as after the transaction completes or when no active transaction requires a particular version.

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Q.4 What happens to uncommitted transactions when using Row Versioning?

Uncommitted transactions generate row versions for modified rows. These row versions are only visible to the transaction that created them. Other transactions will continue to read the committed version of the data.

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Q.5 How do you monitor and troubleshoot row versioning-related issues in SQL Server?

To monitor row versioning, you can use various Dynamic Management Views (DMVs) like sys.dm_tran_version_store_space_usage and sys.dm_tran_active_snapshot_database_transactions. To troubleshoot issues related to version store space in tempdb, consider monitoring tempdb usage and adjusting the tempdb configuration based on usage patterns.

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Q.6 What is a Trigger in SQL Server?

A Trigger is a database object that automatically executes in response to specific DML (Data Manipulation Language) events, such as INSERT, UPDATE, or DELETE operations, on a table.

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Q.7 How many types of Triggers does SQL Server support, and what are they?

SQL Server supports two types of Triggers: i. DML Triggers: These fire in response to INSERT, UPDATE, or DELETE operations. ii. DDL Triggers: These fire in response to Data Definition Language (DDL) events, such as CREATE, ALTER, or DROP statements.

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Q.8 What are the benefits of using Triggers in SQL Server?

Triggers are useful for enforcing data integrity rules, auditing changes to data, maintaining historical records, and automating tasks that need to occur in response to specific database events.

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Q.9 How can you create a Trigger in SQL Server?

To create a Trigger, use the CREATE TRIGGER statement and specify the Trigger name, event(s) to fire the Trigger, and the action(s) to be performed when the Trigger is triggered.

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Q.10 Explain the differences between "AFTER" and "INSTEAD OF" Triggers in SQL Server.

"AFTER" Triggers fire after the DML event completes and data changes have been made. "INSTEAD OF" Triggers, on the other hand, fire instead of the actual DML event, allowing the Trigger to replace or modify the original operation.

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Q.11 What are the potential performance considerations when using Triggers?

Triggers can introduce overhead on DML operations, as they add additional processing to the event they are triggered on. Careful design and optimization are essential to minimize performance impacts.

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Q.12 How can you view existing Triggers in SQL Server?

To view existing Triggers, you can query the sys.triggers catalog view or use SQL Server Management Studio (SSMS) to explore the database objects.

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Q.13 Can a Trigger be disabled or dropped? If so, how?

Yes, a Trigger can be disabled using the "DISABLE TRIGGER" statement, and it can be dropped using the "DROP TRIGGER" statement. Disabling a Trigger prevents it from firing, while dropping removes it from the database entirely.

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Q.14 What is a nested Trigger in SQL Server, and are there any limitations on using them?

A nested Trigger is a Trigger that fires another Trigger. SQL Server allows up to 32 levels of nested Triggers, but it is essential to be cautious about potential cascading actions and the risk of infinite loops.

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Q.15 How can you handle errors in a Trigger in SQL Server?

To handle errors in a Trigger, you can use the TRY...CATCH construct in T-SQL. This allows you to capture and manage exceptions, providing error logging and custom error handling within the Trigger.

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Q.16 What is Partitioning in SQL Server?

Partitioning is a database design technique that involves dividing large database tables into smaller, more manageable segments called partitions based on specific criteria like ranges or lists.

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Q.17 What are the benefits of using Partitioning in SQL Server?

Partitioning offers several benefits, including improved query performance, easier data management, faster data loading and archiving, and enhanced index maintenance.

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Q.18 How can you implement Partitioning in SQL Server?

Partitioning can be implemented at the table level by defining a partition function that specifies how data is divided into partitions and a partition scheme that maps partitions to filegroups.

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Q.19 What are the different types of Partitioning in SQL Server?

SQL Server supports two types of Partitioning: i. Range Partitioning: Data is partitioned based on a specified range of values (e.g., dates or numeric values). ii. List Partitioning: Data is partitioned based on specific values from a column (e.g., categories or regions).

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Q.20 How can you switch partitions in SQL Server?

Partition switching is a technique to move data quickly between tables by using ALTER TABLE...SWITCH. It is beneficial for data loading, archiving, and other bulk data operations.

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Q.21 What is the importance of choosing an appropriate Partitioning Key?

The Partitioning Key is crucial as it determines how data is distributed across partitions. Choosing an inappropriate key may lead to data skew and uneven distribution, impacting performance and query optimization.

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Q.22 How can you identify and monitor Partitioned Tables in SQL Server?

You can query system catalog views such as sys.tables and sys.indexes, along with partition-specific DMVs like sys.partition_functions and sys.partition_schemes, to identify and monitor partitioned tables.

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Q.23 Can you add or remove partitions from an existing Partitioned Table?

Yes, you can add or remove partitions to accommodate changing data needs. However, this requires careful planning and maintenance to ensure the data distribution remains optimal.

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Q.24 What are the limitations or considerations for using Partitioning in SQL Server?

Some considerations include the need for Enterprise Edition or higher, careful design of partitioning keys, impact on non-partitioned indexes, and potential maintenance complexity.

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Q.25 How can you perform data maintenance on Partitioned Tables efficiently?

Data maintenance can be efficiently performed using partition-level operations such as SWITCH, SPLIT, MERGE, or TRUNCATE, which allow for focused operations on specific partitions rather than the entire table.

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Q.26 What is an Index in SQL Server?

An index is a database object that improves query performance by providing quick access to data based on the values in one or more columns of a table.

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Q.27 How does an Index work in SQL Server?

An index works like a lookup table, containing a sorted list of key values and their corresponding pointers to the actual data rows. It allows SQL Server to locate data more efficiently during query execution.

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Q.28 What are the benefits of using Indexes in SQL Server?

Indexes improve query performance by reducing the number of rows that need to be scanned, resulting in faster data retrieval for SELECT, JOIN, and WHERE clauses.

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Q.29 What are the main types of Indexes in SQL Server, and how do they differ?

SQL Server supports several types of indexes, including Clustered Indexes (physical table ordering), Non-Clustered Indexes (separate structure with pointers), and Filtered Indexes (indexes on a subset of data based on a condition).

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Q.30 When would you choose a Clustered Index over a Non-Clustered Index, and vice versa?

A Clustered Index is suitable for columns that are frequently used for sorting and range queries, as it physically orders the data in the table. Non-Clustered Indexes are preferred for columns with frequent search and join operations.

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Q.31 What is the impact of Indexing on data modification operations like INSERT, UPDATE, and DELETE?

While indexes improve read performance, they can slow down data modification operations, as each change to indexed columns requires updating the index data structure as well.

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Q.32 How can you identify and resolve Index Fragmentation in SQL Server?

Index Fragmentation occurs when data pages are not contiguous. It can be identified using the sys.dm_db_index_physical_stats DMV, and it can be resolved by rebuilding or reorganizing indexes using ALTER INDEX statements.

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Q.33 What are the best practices for Index Maintenance in SQL Server?

Some best practices include regular index defragmentation, updating index statistics using the UPDATE STATISTICS command, avoiding over-indexing, and considering filtered indexes for specific scenarios.

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Q.34 How can you determine which Indexes to create for a specific table or query workload?

You can use SQL Server's Query Store, execution plans, and Dynamic Management Views (DMVs) like sys.dm_db_missing_index_details to identify missing or underutilized indexes for specific queries.

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Q.35 Can Indexes be created on computed columns or views in SQL Server?

Yes, SQL Server allows you to create indexes on computed columns and indexed views. However, keep in mind that indexing computed columns can introduce additional overhead on data modification operations.

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Q.36 What is Performance Tuning in SQL Server?

Performance Tuning in SQL Server refers to the process of optimizing the database and queries to improve system response time, reduce resource utilization, and enhance overall database performance.

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Q.37 How can you identify performance bottlenecks in SQL Server?

Performance bottlenecks can be identified using SQL Server Profiler, Extended Events, Dynamic Management Views (DMVs), and SQL Server Management Studio (SSMS) reports like Query Performance Insights.

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Q.38 What are some common performance bottlenecks in SQL Server and how can you address them?

Common bottlenecks include CPU contention, I/O latency, locking and blocking, and suboptimal query execution plans. Addressing them involves proper indexing, optimizing queries, increasing hardware resources, and improving query design.

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Q.39 How can you improve query performance in SQL Server?

Query performance can be improved by creating appropriate indexes, using indexed views or materialized views, updating statistics, rewriting complex queries, and avoiding excessive use of functions in WHERE clauses.

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Q.40 What are Execution Plans in SQL Server, and how can you use them for performance tuning?

Execution Plans are graphical representations of how SQL Server executes a query. You can analyze them using SSMS or tools like SQL Server Query Store to identify performance issues and optimize query performance.

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Q.41 What is the purpose of the SQL Server Query Store in performance tuning?

The Query Store captures and retains query execution plans and performance statistics, allowing you to analyze query performance over time, identify regressions, and force optimal plans if necessary.

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Q.42 How can you optimize disk I/O in SQL Server for better performance?

Disk I/O can be optimized by placing data and log files on separate physical drives, using RAID configurations for fault tolerance and performance, and using Solid-State Drives (SSDs) for high-performance workloads.

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Q.43 What is Parameter Sniffing, and how can it impact query performance?

Parameter Sniffing occurs when SQL Server compiles and optimizes a query plan based on the first set of parameters used. It can lead to suboptimal plans for subsequent executions with different parameters. Techniques like OPTION (RECOMPILE) or plan guides can address parameter sniffing issues.

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Q.44 How can you monitor and manage SQL Server memory to improve performance?

Monitoring SQL Server memory usage can be done using DMVs like sys.dm_os_performance_counters and sys.dm_os_memory_clerks. You can optimize memory settings like Max Server Memory to prevent memory pressure and improve performance.

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Q.45 What are the best practices for ongoing performance monitoring and maintenance in SQL Server?

Regularly monitor system performance using SQL Server DMVs, Profiler, and Extended Events. Implement index maintenance tasks, update statistics, and periodically review and optimize queries to ensure continued performance improvements.

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Q.46 What is the role of SQL Server Configuration Manager?

Following are the roles of a SQL Server Configuration Manager. 1. to start, stop, pause, and restart SQL Server services 2. to configure the use of network protocols to access SQL Server 3. to configure SQL Server Native Client connectivity

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Q.47 Which operating system is not compatible with SQL Server 2008 for installation?

Windows Server NT 4.0 is not compatible with SQL Server 2008

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Q.48 Which software system is needed for SQL Server 2008 installation?

.NET Framework 3.5 is need for SQL Server 2008 installation

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Q.49 What is the minimum amount of memory needed to run SQL Server 2008?

512MB of memory is at least needed to run SQL Server 2008

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Q.50 Which shape is used to represent an entity in E-R diagram?

Rectangle is used to represent an entity in E-R diagram.

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Q.51 What is an index in SQL Server, and how does it improve performance?

An index is a database object that speeds up query execution by creating a copy of a subset of data and organizing it for quick retrieval. It helps improve performance by reducing the number of data pages that need to be read when searching for specific data, thus minimizing disk I/O and query response time.

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Q.52 What are the main types of indexes in SQL Server?

SQL Server supports clustered and non-clustered indexes. A clustered index determines the physical order of data on disk, while a non-clustered index creates a separate structure that points to the actual data.

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Q.53 When should you use a clustered index, and when should you use a non-clustered index?

A clustered index is ideal for columns that are frequently used for range-based queries or for primary key columns. A non-clustered index is more suitable for columns that are frequently searched but do not offer sequential access.

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Q.54 How does the Fill Factor property impact index performance?

The Fill Factor property controls the percentage of space to be left empty on each data page when creating or rebuilding an index. A lower Fill Factor reduces data fragmentation, leading to better performance on insert operations but may require more space. A higher Fill Factor optimizes read performance but increases fragmentation.

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Q.55 What is the difference between INCLUDE and COVERING indexes?

Both INCLUDE and COVERING indexes are used to cover query columns not present in the index key. However, an INCLUDE index includes additional non-key columns in the leaf level, while a COVERING index refers to a non-clustered index that covers all columns in a query, avoiding the need to access the base table.

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Q.56 How can you identify index fragmentation, and why is it important to address it?

Index fragmentation can be identified using the sys.dm_db_index_physical_stats DMV. It is essential to address fragmentation as it leads to increased disk I/O and reduced query performance. Regular index maintenance, such as rebuilding or reorganizing indexes, helps improve performance.

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Q.57 What are filtered indexes, and when should you use them?

Filtered indexes are non-clustered indexes with a WHERE clause that filters the data stored in the index. They are useful for improving query performance on a specific subset of data that meets the filter condition, reducing the index size and maintenance overhead.

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Q.58 Explain the concept of index statistics and their importance.

Index statistics contain information about the distribution of data in an index. SQL Server's query optimizer uses these statistics to create optimal query plans. Outdated or inaccurate statistics can lead to poor query performance, so it's crucial to keep them up-to-date using the UPDATE STATISTICS command.

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Q.59 Can you have multiple indexes on the same column, and if so, when would you use them?

Yes, you can have multiple indexes on the same column. Multiple indexes might be useful when you have different types of queries that access the same column with varying WHERE clauses, sorting orders, or join conditions, and you want to optimize the performance for each specific scenario.

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Q.60 How do you monitor index usage and performance in SQL Server?

SQL Server provides various Dynamic Management Views (DMVs) and Performance Monitor counters to monitor index usage and performance. DMVs like sys.dm_db_index_usage_stats and sys.dm_db_index_operational_stats offer valuable insights into index activity and can help identify unused or underperforming indexes. Performance Monitor counters like "SQLServer:Access Methods - Indexes Searches/sec" can also provide valuable performance metrics.

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Q.61 What is index optimization, and why is it essential for SQL Server performance?

Index optimization refers to the process of improving the efficiency and effectiveness of indexes to enhance query performance. It is crucial because well-optimized indexes reduce disk I/O, minimize query execution time, and improve overall database performance.

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Q.62 How do you identify poorly performing indexes in SQL Server?

Poorly performing indexes can be identified using various methods such as analyzing query plans, monitoring wait statistics, reviewing Dynamic Management Views (DMVs) like sys.dm_db_index_operational_stats, and tracking index fragmentation using sys.dm_db_index_physical_stats.

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Q.63 What are the potential reasons for index fragmentation, and how can you address it?

Index fragmentation occurs due to data modifications (INSERT, UPDATE, DELETE) that cause pages to split or become less contiguous. You can address fragmentation by rebuilding or reorganizing indexes. Rebuilding rebuilds the entire index, while reorganizing defragments it by physically reordering the leaf-level pages.

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Q.64 When would you choose to rebuild an index versus reorganize it?

You would rebuild an index when it has significant fragmentation (e.g., fragmentation level exceeds 30%) or when there are substantial changes to the index definition. Reorganizing is preferred for mild fragmentation, as it is a lighter-weight operation that keeps the index online during the process.

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Q.65 What is the impact of the FILLFACTOR setting on index optimization?

The FILLFACTOR setting determines the amount of free space reserved on each data page when creating or rebuilding an index. A lower FILLFACTOR reduces fragmentation and improves write performance but increases the index's size. A higher FILLFACTOR optimizes read performance at the expense of increased fragmentation.

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Q.66 How can you determine the right indexes to create for a specific query workload?

You can use SQL Server's Query Store, execution plans, and DMVs like sys.dm_db_missing_index_details to identify missing indexes for specific queries. However, it's essential to carefully analyze and validate the recommendations before creating new indexes to avoid index proliferation.

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Q.67 What is the significance of covering indexes, and how can they improve query performance?

Covering indexes include all the columns required to satisfy a query, so the database engine can retrieve data directly from the index without accessing the base table. This reduces disk I/O and can significantly enhance query performance by eliminating the need for additional table lookups.

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Q.68 How can you maintain index optimization over time to ensure continued performance benefits?

Regular index maintenance is critical for sustained performance. Schedule periodic index rebuilds and reorganizations based on the level of fragmentation and query workload changes. Additionally, keep index statistics up-to-date using the UPDATE STATISTICS command to enable the query optimizer to make informed decisions.

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Q.69 What are the best practices for index optimization in SQL Server?

Some best practices include: Designing indexes based on query patterns and workload analysis. Avoiding over-indexing to prevent excessive overhead on data modification operations. Regularly monitoring index fragmentation and rebuilding/reorganizing indexes accordingly. Verifying the impact of new indexes before implementing them in production.

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Q.70 How can you measure the performance improvement achieved through index optimization?

To measure the performance improvement, you can use SQL Server's performance monitoring tools, such as Performance Monitor and Extended Events, to track relevant performance counters like "Batch Requests/Sec," "Average Disk Sec/Read," and "Average Disk Sec/Write." Additionally, compare query execution times and resource utilization before and after index optimization. Keep in mind that index optimization is an ongoing process and may require adjustments over time based on changes in the database workload and usage patterns.

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Q.71 What is a User-Defined Function (UDF) in SQL Server?

A User-Defined Function (UDF) is a custom, reusable program written in T-SQL that accepts parameters, performs a specific operation, and returns a single value or a table result set.

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Q.72 What are the main types of User-Defined Functions in SQL Server?

There are three main types of UDFs in SQL Server: i. Scalar functions: Return a single scalar value, such as an integer or string. ii. Inline table-valued functions: Return a table result set using the "RETURN TABLE" syntax. iii. Multi-statement table-valued functions: Return a table result set using "INSERT" statements to build the result.

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Q.73 How are scalar functions different from table-valued functions?

Scalar functions return a single value, whereas table-valued functions return a table result set. Scalar functions are typically used in expressions, while table-valued functions are used to return multiple rows of data.

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Q.74 What are the advantages of using User-Defined Functions?

The advantages of using UDFs include code reusability, modular design, improved readability, and the ability to encapsulate complex logic in a single function.

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Q.75 Can User-Defined Functions modify data in the database?

No, by default, UDFs are not allowed to modify data in the database. They are intended for read-only operations. Attempting to modify data within a UDF will result in an error.

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Q.76 What are the considerations when using UDFs in queries?

While UDFs offer flexibility, they can impact query performance if not used judiciously. It's important to consider the performance implications and avoid using UDFs in JOIN and WHERE clauses, as they might hinder the query optimizer's ability to optimize the execution plan.

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Q.77 How can you create and use a Scalar User-Defined Function in SQL Server?

To create a scalar UDF, use the "CREATE FUNCTION" statement. To use it, simply call the function within a SELECT or WHERE clause, providing the necessary parameters.

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Q.78 What are the restrictions on using User-Defined Functions in SQL Server?

UDFs have several limitations, including the inability to perform certain actions like modifying data, using non-deterministic functions like GETDATE() in a scalar UDF, and using side-effecting functions like PRINT.

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Q.79 When would you choose an Inline table-valued function over a Multi-statement table-valued function?

An Inline table-valued function is preferred when the logic is relatively simple and can be expressed in a single SELECT statement. Multi-statement table-valued functions are used when the logic requires more complex processing using multiple SQL statements.

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Q.80 How can you improve the performance of User-Defined Functions in SQL Server?

To improve UDF performance, consider using them sparingly and avoiding them in JOIN and WHERE clauses. Additionally, create indexes on columns used in UDFs to enhance query performance and limit the use of non-deterministic functions within scalar UDFs. UDFs can be powerful tools when used appropriately, but understanding their limitations and performance considerations is essential for effective database development and optimization.

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Q.81 What is Transact-SQL (T-SQL)?

Transact-SQL (T-SQL) is Microsoft's proprietary extension of SQL, used for querying and managing data in Microsoft SQL Server. It includes additional features like variables, flow control statements, error handling, and stored procedures.

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Q.82 How can you declare and use variables in T-SQL?

Variables in T-SQL are declared using the "@variable_name" syntax. For example, to declare an integer variable and use it in a query: DECLARE @MyVariable INT; SET @MyVariable = 42; SELECT * FROM MyTable WHERE ColumnName = @MyVariable; .

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Q.83 What are the different types of triggers available in T-SQL?

T-SQL supports two types of triggers: AFTER triggers, which fire after a DML (Data Manipulation Language) operation, and INSTEAD OF triggers, which replace the original DML operation.

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Q.84 How can you handle errors in T-SQL, and what are the benefits of using TRY...CATCH blocks?

T-SQL provides error handling using TRY...CATCH blocks. TRY...CATCH allows you to handle errors gracefully by capturing exceptions and executing specific error-handling code. This helps in managing and logging errors effectively.

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Q.85 How can you create temporary tables in T-SQL, and what are their benefits?

Temporary tables in T-SQL can be created using the "#temp_table" syntax. They are useful for storing intermediate results within a session and can significantly improve performance by reducing repeated data access and calculations.

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Q.86 What is the difference between a stored procedure and a user-defined function in T-SQL?

Stored procedures are T-SQL programs that can contain DML and DDL statements, whereas user-defined functions return scalar values or table result sets and cannot contain DML operations. Stored procedures do not necessarily return values, while functions must return a value.

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Q.87 How can you concatenate strings in T-SQL, and what are the considerations for string concatenation?

In T-SQL, you can concatenate strings using the '+' operator or the CONCAT function. When concatenating large strings in a loop, be cautious of performance issues, and consider using the STRING_AGG function (available in SQL Server 2017 and later) for aggregating string values.

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Q.88 What are Common Table Expressions (CTEs), and how are they useful in T-SQL?

Common Table Expressions (CTEs) are temporary result sets that can be referenced within a SELECT, INSERT, UPDATE, or DELETE statement. CTEs enhance code readability and simplify complex queries by breaking them down into smaller, manageable parts.

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Q.89 What is the purpose of the RANK() function in T-SQL?

The RANK() function is used for ranking rows based on a specified column's values. It assigns a unique rank value to each row, allowing you to identify top-N or bottom-N records based on the ranking criteria.

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Q.90 How can you pivot data in T-SQL using the PIVOT operator?

The PIVOT operator in T-SQL allows you to transform row-level data into columnar data. It is useful for cross-tabulation queries where you want to display data in a more structured, pivoted format. T-SQL is a powerful language that provides a wide range of features for managing and manipulating data in SQL Server. Understanding its syntax and capabilities is crucial for efficient database development and administration.

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Q.91 What is a transaction in SQL Server, and what are its ACID properties?

A transaction is a logical unit of work that contains one or more SQL statements. Transactions follow the ACID properties: Atomicity (all or nothing), Consistency (ensuring data integrity), Isolation (concurrent operations do not interfere), and Durability (committed changes are permanent).

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Q.92 How can you begin a transaction in SQL Server, and how can you end it?

To begin a transaction, use the BEGIN TRANSACTION statement. To end a transaction, you can either COMMIT to save the changes or ROLLBACK to undo them based on the success or failure of the transaction.

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Q.93 What is the default behavior of SQL Server regarding transactions?

By default, SQL Server operates in autocommit mode, where each individual SQL statement is treated as a separate transaction. As soon as a statement is executed, it is committed automatically.

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Q.94 How can you set the isolation level for a transaction, and why is it essential to consider?

The isolation level can be set using the SET TRANSACTION ISOLATION LEVEL statement. It is essential to consider the isolation level to manage data concurrency and prevent issues like dirty reads, non-repeatable reads, and phantom reads.

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Q.95 What is a deadlock in SQL Server, and how can you identify and resolve it?

A deadlock occurs when two or more transactions are waiting for resources locked by each other, resulting in a cycle of dependencies. Deadlocks can be identified using SQL Server's system_health Extended Events session or by monitoring the sys.dm_tran_locks DMV. To resolve deadlocks, consider optimizing queries, reducing transaction size, or using the SET DEADLOCK_PRIORITY option.

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Q.96 What are the implications of using the READ UNCOMMITTED isolation level in SQL Server?

READ UNCOMMITTED is the least restrictive isolation level, allowing dirty reads. While it can improve concurrency, it may lead to inconsistent and inaccurate data retrieval, making it unsuitable for certain critical applications.

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Q.97 How can you handle errors and exceptions in transactions effectively?

You can use the TRY...CATCH construct in T-SQL to handle errors and exceptions gracefully within a transaction. This allows you to capture and manage errors, roll back the transaction if needed, and maintain data integrity.

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Q.98 What is a Savepoint in SQL Server, and how can it be used in transactions?

A Savepoint allows you to set a marker within a transaction to which you can later roll back if needed. It enables partial rollback within a larger transaction and helps in handling complex scenarios where only certain parts of the transaction need to be undone.

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Q.99 How can you monitor and track the status of transactions in SQL Server?

SQL Server provides various DMVs like sys.dm_tran_active_transactions and sys.dm_tran_database_transactions to monitor active transactions. The sys.dm_exec_requests DMV can be used to view information about currently executing queries, including transaction status.

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Q.100 What are the best practices for managing transactions in SQL Server?

Some best practices include: Keeping transactions short and focused on specific tasks. Minimizing the time spent in transactions to reduce locking and blocking. Avoiding nested transactions whenever possible. Implementing proper error handling and rollback mechanisms to maintain data integrity.

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Q.101 What is locking in SQL Server, and why is it necessary?

Locking in SQL Server is a mechanism used to manage concurrent access to shared resources (rows, pages, or tables). It ensures data consistency and prevents data integrity issues that may arise when multiple transactions try to access or modify the same data simultaneously.

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Q.102 What are the main types of locks in SQL Server, and how do they differ?

SQL Server supports various types of locks, including Shared (S) locks, Exclusive (X) locks, Update (U) locks, Intent (I) locks, and more. Shared locks allow multiple transactions to read data simultaneously, while Exclusive locks prevent any other transaction from accessing the locked resource.

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Q.103 How can you monitor lock activity in SQL Server?

Lock activity can be monitored using Dynamic Management Views (DMVs) such as sys.dm_tran_locks and sys.dm_exec_requests. These DMVs provide valuable information about current locks and the status of transactions.

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Q.104 What is a deadlock in SQL Server, and how can you prevent it?

A deadlock occurs when two or more transactions are waiting for resources locked by each other, leading to a cyclic dependency. To prevent deadlocks, use appropriate isolation levels, keep transactions short and focused, and design queries in a way that minimizes locking conflicts.

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Q.105 What is the difference between row-level and page-level locking in SQL Server?

Row-level locking locks individual rows, allowing more concurrent access to data within a page. Page-level locking locks entire data pages, reducing locking overhead but potentially causing more contention for the locked page.

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Q.106 How can you force a particular type of lock in SQL Server?

You can force a particular type of lock using the HOLDLOCK hint or by specifying an appropriate isolation level, such as SERIALIZABLE. However, it is essential to use such hints judiciously, as they may lead to unwanted locking behavior.

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Q.107 What is lock escalation, and how does it impact performance?

Lock escalation is the process by which SQL Server converts multiple fine-grained locks (e.g., row-level) to fewer coarse-grained locks (e.g., page-level or table-level). While it can reduce memory consumption, excessive lock escalation might lead to contention and negatively impact performance.

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Q.108 How can you resolve locking and blocking issues in SQL Server?

To resolve locking and blocking issues, identify the problematic queries and tune them for better performance. You can also consider implementing appropriate isolation levels, using Read Committed Snapshot Isolation (RCSI), or applying query hints like NOLOCK (READ UNCOMMITTED) with caution.

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Q.109 Explain the concept of lock compatibility in SQL Server.

Lock compatibility determines whether a transaction can acquire a particular lock on a resource without waiting or causing a deadlock. Compatible locks are those that can coexist without conflicting, while incompatible locks would cause blocking or a deadlock.

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Q.110 What are the best practices for optimizing locking in SQL Server?

Some best practices include: Keeping transactions short and focused. Using the appropriate isolation level for each transaction. Avoiding unnecessary hints or forcing locks unless required. Identifying and addressing long-running queries that cause excessive locking. Understanding locking mechanisms in SQL Server is crucial for managing concurrent access to data effectively and ensuring optimal database performance.

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Q.111 What are Isolation Levels in SQL Server, and why are they important?

Isolation Levels define how transactions interact with each other regarding concurrent access to data. They are essential to maintain data consistency and prevent concurrency-related issues like dirty reads, non-repeatable reads, and phantom reads.

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Q.112 How many Isolation Levels does SQL Server support, and what are they?

SQL Server supports four standard Isolation Levels: i. READ UNCOMMITTED ii. READ COMMITTED iii. REPEATABLE READ iv. SERIALIZABLE.

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Q.113 What is the default Isolation Level in SQL Server, and how can you change it?

The default Isolation Level in SQL Server is READ COMMITTED. You can change the Isolation Level for a session using the SET TRANSACTION ISOLATION LEVEL statement.

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Q.114 Explain the READ UNCOMMITTED Isolation Level and its implications.

READ UNCOMMITTED allows transactions to read uncommitted changes made by other transactions. While it improves concurrency, it can lead to dirty reads, non-repeatable reads, and phantom reads, making it less suitable for critical operations.

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Q.115 What is the difference between READ COMMITTED and REPEATABLE READ Isolation Levels?

READ COMMITTED allows a transaction to read only committed data, while REPEATABLE READ locks the data being read to ensure consistency during the transaction. REPEATABLE READ prevents other transactions from modifying or inserting data read by the current transaction.

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Q.116 Explain the SERIALIZABLE Isolation Level and its impact on concurrency.

SERIALIZABLE locks all the data read and prevents other transactions from modifying or inserting data until the transaction is complete. It provides the highest level of data consistency but may lead to increased blocking and reduced concurrency.

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Q.117 How can you choose the appropriate Isolation Level for a transaction?

The appropriate Isolation Level depends on the specific requirements of the transaction and the desired balance between data consistency and concurrency. Understanding the isolation behaviors and analyzing the impact on the application is crucial in selecting the right level.

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Q.118 What is the purpose of the SNAPSHOT Isolation Level in SQL Server?

SNAPSHOT Isolation Level allows transactions to read a version of data as of the start of the transaction, avoiding blocking and enabling non-blocking read consistency. It uses row versioning to achieve this behavior.

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Q.119 How can you enable and use Read Committed Snapshot Isolation (RCSI) in SQL Server?

RCSI can be enabled using the ALTER DATABASE command with the ALLOW_SNAPSHOT_ISOLATION option. Once enabled, the Read Committed transactions will use row versioning, similar to SNAPSHOT Isolation, to improve concurrency and avoid blocking.

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Q.120 What are the potential performance trade-offs when using higher Isolation Levels?

Higher Isolation Levels like REPEATABLE READ and SERIALIZABLE might lead to increased blocking and contention due to more aggressive locking. It is essential to weigh the benefits of data consistency against potential performance implications when choosing an Isolation Level. Understanding Isolation Levels in SQL Server is crucial for managing data consistency and concurrency effectively. Properly configuring Isolation Levels based on application requirements can significantly impact the database's performance and reliability.

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Q.121 What is Row Versioning in SQL Server?

Row Versioning is a mechanism used by SQL Server to manage concurrency and provide read consistency in multi-user environments. It involves creating and storing versions of data rows to support non-blocking reads during transactions.

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Q.122 How does Row Versioning work in SQL Server?

SQL Server uses row versioning to maintain previous versions of data when a transaction modifies a row. Read operations during concurrent transactions can access the appropriate version without being blocked.

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Q.123 What are the two main implementations of Row Versioning in SQL Server?

SQL Server uses two implementations of Row Versioning: Read Committed Snapshot Isolation (RCSI) and Snapshot Isolation. RCSI uses row versioning to provide read consistency for Read Committed transactions, while Snapshot Isolation provides full row versioning for all transactions.

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Q.124 How can you enable Read Committed Snapshot Isolation (RCSI) in SQL Server?

RCSI can be enabled at the database level using the ALTER DATABASE command with the ALLOW_SNAPSHOT_ISOLATION option set to ON.

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Q.125 What are the benefits of using Read Committed Snapshot Isolation (RCSI)?

RCSI allows Read Committed transactions to read data without blocking other transactions and provides consistent read views. This improves concurrency and reduces contention in multi-user environments.

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