Top 30 SQL Interview Questions and Answers
1. What is SQL?
SQL (Structured Query Language) is used to communicate with databases. It lets you create, read,
update, and delete data (CRUD operations).
2. What are the types of SQL statements?
1. DDL (Data Definition Language): CREATE, ALTER, DROP 2. DML (Data Manipulation
Language): INSERT, UPDATE, DELETE 3. DCL (Data Control Language): GRANT, REVOKE 4.
TCL (Transaction Control Language): COMMIT, ROLLBACK 5. DQL (Data Query Language):
SELECT
3. Difference between DELETE, TRUNCATE, and DROP?
DELETE: Removes rows one by one (can use WHERE). TRUNCATE: Removes all rows, resets
identity, faster. DROP: Removes the entire table structure.
4. What is a Primary Key?
A primary key uniquely identifies each record in a table. It cannot have NULL or duplicate values.
5. What is a Foreign Key?
A foreign key links two tables and enforces referential integrity between them.
6. Difference between WHERE and HAVING?
WHERE filters rows before grouping. HAVING filters groups after aggregation.
7. What are Joins?
Joins combine data from multiple tables based on related columns. Types: INNER, LEFT, RIGHT,
FULL, CROSS JOIN.
8. What is Normalization?
Normalization organizes data to reduce redundancy. Forms: 1NF, 2NF, 3NF, BCNF.
9. What is Denormalization?
The process of adding redundancy to improve read performance.
10. What is an Index?
Indexes improve query speed but slow down insert/update. Clustered index sorts actual data;
non-clustered stores pointers.
11. What are Views?
Views are virtual tables based on queries. They don’t store data physically.
