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SQL Interview Questions and Answers

1. What is SQL?​

SQL (Structured Query Language) is a standard language for managing and manipulating relational databases. It is used to query, insert, update, and delete data in databases.

2. What are the different types of SQL commands?​

  • DDL (Data Definition Language): CREATE, ALTER, DROP
  • DML (Data Manipulation Language): SELECT, INSERT, UPDATE, DELETE
  • DCL (Data Control Language): GRANT, REVOKE
  • TCL (Transaction Control Language): COMMIT, ROLLBACK, SAVEPOINT

3. Explain the difference between WHERE and HAVING clauses.​

  • WHERE filters rows before grouping.
  • HAVING filters groups after aggregation.

Example:

SELECT department, AVG(salary) FROM employees WHERE salary > 50000 GROUP BY department HAVING AVG(salary) > 60000;

4. What is a JOIN in SQL?​

A JOIN combines rows from two or more tables based on a related column. Types include INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL OUTER JOIN.

Example:

SELECT employees.name, departments.dept_name FROM employees INNER JOIN departments ON employees.dept_id = departments.id;

5. What is a subquery?​

A subquery is a query nested inside another query. It can be used in SELECT, FROM, WHERE clauses.

Example:

SELECT name FROM employees WHERE salary > (SELECT AVG(salary) FROM employees);

6. Explain normalization in databases.​

Normalization is the process of organizing data to reduce redundancy and improve data integrity. It involves dividing large tables into smaller, related tables and defining relationships.

7. What is an index in SQL?​

An index is a database object that improves the speed of data retrieval operations on a table. It works like a pointer to data in a table.

8. How do you handle NULL values in SQL?​

Use IS NULL or IS NOT NULL in WHERE clauses. Functions like COALESCE or NVL can replace NULL with default values.

Example:

SELECT name, COALESCE(phone, 'N/A') FROM employees;

9. What is a primary key?​

A primary key is a unique identifier for each record in a table. It cannot contain NULL values and must be unique.

10. Explain ACID properties in transactions.​

  • Atomicity: All operations in a transaction succeed or none do.
  • Consistency: Database remains in a consistent state.
  • Isolation: Transactions are isolated from each other.
  • Durability: Changes are permanent once committed.