SQL Server Interview Questions and Answers for 10 years experience

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100 SQL Server Interview Questions & Answers (10+ Years Experience)

  1. What are the different types of joins in SQL Server? Explain with examples.

    • Answer: SQL Server supports several join types: INNER JOIN (returns rows only when there is a match in both tables), LEFT (OUTER) JOIN (returns all rows from the left table and matching rows from the right table; NULLs for non-matches), RIGHT (OUTER) JOIN (returns all rows from the right table and matching rows from the left table; NULLs for non-matches), FULL (OUTER) JOIN (returns all rows from both tables; NULLs for non-matches in either table), and CROSS JOIN (returns the Cartesian product of the tables). Example: `SELECT * FROM Employees INNER JOIN Departments ON Employees.DepartmentID = Departments.ID;` This would only show employees who belong to a department listed in the Departments table.

  2. Explain indexing in SQL Server and its benefits. What are different types of indexes?

    • Answer: Indexes are data structures that improve the speed of data retrieval operations on a database table. They work similarly to an index in a book. Benefits include faster query execution, improved performance of WHERE and JOIN clauses, and reduced I/O operations. Types of indexes include clustered (physically sorts the table data) and non-clustered (creates a separate structure pointing to the table data), unique (ensures uniqueness of indexed column), and filtered (indexes only a subset of data based on a condition).

  3. How do you handle deadlocks in SQL Server?

    • Answer: Deadlocks occur when two or more processes are blocked indefinitely, waiting for each other to release resources. Strategies to handle them include minimizing transaction duration, setting appropriate isolation levels (Read Uncommitted can reduce deadlocks but increases the risk of dirty reads), using appropriate locking hints (if necessary), designing transactions carefully to reduce resource contention, and reviewing the deadlock graph in SQL Server Profiler or using extended events to identify patterns and optimize code.

  4. What are stored procedures and why are they used?

    • Answer: Stored procedures are pre-compiled SQL code that resides on the database server. They offer several advantages: improved performance (pre-compilation), enhanced security (restrict access to underlying tables), code reusability, reduced network traffic (single call instead of multiple SQL statements), and better maintainability.

  5. Explain different transaction isolation levels in SQL Server.

    • Answer: Transaction isolation levels control the degree to which concurrent transactions are isolated from each other. They include Read Uncommitted (dirty reads possible), Read Committed (non-repeatable reads possible), Repeatable Read (phantom reads possible), and Serializable (prevents all concurrency issues but may impact performance). Choosing the appropriate level balances data integrity with performance.

  6. How do you optimize SQL queries for performance?

    • Answer: Query optimization involves several techniques: using appropriate indexes, writing efficient SQL code (avoiding SELECT *), optimizing joins (choosing the right join type), using set-based operations instead of cursors, analyzing execution plans using SQL Server Profiler or Database Engine Tuning Advisor, and ensuring sufficient server resources (memory, CPU, I/O).

  7. What are common performance bottlenecks in SQL Server?

    • Answer: Common performance bottlenecks include slow queries (lack of indexes, inefficient code), insufficient server resources (CPU, memory, disk I/O), blocking and deadlocks, poorly designed database schema, and lack of query optimization.

  8. Explain the concept of partitioning in SQL Server.

    • Answer: Partitioning horizontally divides a large table into smaller, more manageable pieces. This improves query performance on large tables by allowing queries to access only the relevant partitions. It also facilitates easier table maintenance, backup, and recovery operations.

  9. What are triggers in SQL Server? Give an example.

    • Answer: Triggers are stored procedures that automatically execute in response to certain events on a particular table or view. For instance, an INSERT trigger can automatically populate an audit table whenever a new row is added to a table. Example: `CREATE TRIGGER AuditInsert ON Employees AFTER INSERT AS ...`

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