SQL Server Interview Questions and Answers for 5 years experience

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

   • Answer: SQL Server supports various JOIN types: INNER JOIN (returns rows only when there's a match in both tables), LEFT (OUTER) JOIN (returns all rows from the left table and matching rows from the right), RIGHT (OUTER) JOIN (returns all rows from the right table and matching rows from the left), and FULL (OUTER) JOIN (returns all rows when there is a match in either left or right table). Examples would involve two tables, say Employees and Departments, joined on EmployeeID and DepartmentID respectively. Each join type would produce a different result set based on the matching criteria.

2. Explain the difference between clustered and non-clustered indexes.

   • Answer: A clustered index defines the physical order of data rows in a table. A table can have only one clustered index. A non-clustered index is a separate structure that points to the data rows; a table can have multiple non-clustered indexes. Clustered indexes improve data retrieval speed for queries based on the indexed columns, but may slow down data modification operations. Non-clustered indexes are faster for lookups than full table scans, but don't affect the physical order of the data.

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

   • Answer: Stored procedures are pre-compiled SQL code blocks stored in the database. They improve performance by reducing parsing overhead and offer better security by controlling access to data through permissions. They also promote code reusability and maintainability.

4. Explain different types of transactions and their properties (ACID).

   • Answer: Transactions are sequences of operations treated as a single unit of work. ACID properties ensure data integrity: Atomicity (all or nothing), Consistency (data remains valid), Isolation (concurrent transactions don't interfere), Durability (committed transactions survive failures). Different transaction types include implicit and explicit transactions, managed by the database system or explicitly defined by the developer.

5. How do you handle deadlocks in SQL Server?

   • Answer: Deadlocks occur when two or more transactions are blocked indefinitely, waiting for each other to release locks. Techniques to handle deadlocks include minimizing lock duration (using shorter transactions), avoiding long-running transactions, using appropriate isolation levels, and monitoring and troubleshooting deadlocks using SQL Server's deadlock detection mechanisms. The database system typically detects and resolves deadlocks automatically by rolling back one of the transactions.

6. What are different isolation levels in SQL Server and their implications?

   • Answer: SQL Server offers various isolation levels (Read Uncommitted, Read Committed, Repeatable Read, Serializable) controlling the level of concurrency and consistency. Higher isolation levels (like Serializable) minimize concurrency issues but reduce performance, while lower levels (like Read Uncommitted) increase concurrency but risk reading uncommitted data (dirty reads).

7. Explain the concept of indexing in SQL Server.

   • Answer: Indexes are data structures that improve the speed of data retrieval operations on a database table. They provide a quick lookup mechanism to locate specific data rows without performing a full table scan. Different types of indexes exist, including clustered and non-clustered, B-tree and others, each with different performance characteristics.

8. What are triggers and how are they used?

   • Answer: Triggers are stored procedures automatically executed in response to certain events on a particular table or view (like INSERT, UPDATE, DELETE). They enforce business rules, maintain data integrity, and automate tasks. They can be defined to run before or after the triggering event.

9. How do you optimize SQL queries for performance?

   • Answer: Query optimization involves various techniques: using appropriate indexes, writing efficient SQL code (avoiding wildcard leading characters in LIKE clauses), optimizing joins, using set-based operations instead of row-by-row processing, analyzing query execution plans (using SQL Server Profiler or similar tools), and ensuring proper database design.

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