SQL Server Interview Questions and Answers for 7 years experience

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SQL Server Interview Questions and Answers

  1. What are the different types of joins in SQL Server? Explain each with an example.

    • 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-matching), RIGHT (OUTER) JOIN (returns all rows from the right table and matching rows from the left table; NULLs for non-matching), FULL (OUTER) JOIN (returns all rows from both tables; NULLs for non-matching). Example: `SELECT * FROM Employees INNER JOIN Departments ON Employees.DepartmentID = Departments.ID;` (Inner Join)

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

    • Answer: A clustered index determines 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 specific queries, while non-clustered indexes speed up searches on specific columns without affecting the physical data order.

  3. What are stored procedures? What are their advantages?

    • Answer: Stored procedures are pre-compiled SQL code blocks stored in the database. Advantages include: improved performance (pre-compilation), enhanced security (control over data access), code reusability, reduced network traffic (single call instead of multiple statements), and easier maintenance.

  4. How do you handle transactions in SQL Server?

    • Answer: Transactions are used to manage sets of operations as a single unit of work. They guarantee atomicity (all or nothing), consistency, isolation, and durability (ACID properties). They are managed using `BEGIN TRANSACTION`, `COMMIT TRANSACTION`, and `ROLLBACK TRANSACTION` statements.

  5. Explain different data types in SQL Server.

    • Answer: SQL Server offers various data types: `INT`, `BIGINT` (integers), `FLOAT`, `DECIMAL` (floating-point and fixed-point numbers), `VARCHAR`, `NVARCHAR`, `CHAR`, `NCHAR` (character strings), `DATE`, `DATETIME`, `DATETIME2`, `TIME` (dates and times), `BIT` (boolean), `UNIQUEIDENTIFIER` (GUIDs), etc. The choice depends on the nature and size of the data being stored.

  6. What are triggers? Give an example of when you would use one.

    • Answer: Triggers are stored procedures automatically executed in response to specific events on a particular table or view. For example, an `AFTER INSERT` trigger could automatically update an audit table whenever a new row is inserted into the main table, logging the insertion details.

  7. How do you optimize SQL queries for better performance?

    • Answer: Optimization techniques include: creating appropriate indexes, using efficient joins, avoiding `SELECT *`, using `WHERE` clauses effectively, optimizing data types, writing efficient stored procedures, using query hints judiciously, and analyzing execution plans with SQL Server Profiler or Database Engine Tuning Advisor.

  8. Explain the concept of normalization in database design.

    • Answer: Normalization is the process of organizing data to reduce redundancy and improve data integrity. It involves applying normal forms (1NF, 2NF, 3NF, BCNF, etc.), each aiming to reduce data redundancy and improve data consistency. Proper normalization makes databases more efficient and easier to maintain.

  9. What are user-defined functions (UDFs)? What are the different types?

    • Answer: UDFs are reusable blocks of SQL code that perform a specific task and return a value. Types include scalar UDFs (return a single value), table-valued UDFs (return a result set), and inline table-valued functions (more efficient for simple operations).

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