Question 1

What are the main types of SQL commands?

Accepted Answer

SQL commands are grouped into DDL (Data Definition Language), DML (Data Manipulation Language), DCL (Data Control Language), and TCL (Transaction Control Language).

Question 2

What's the difference between DELETE and TRUNCATE?

Accepted Answer

DELETE removes specific rows and can be rolled back. TRUNCATE removes all rows, resets the identity counter, is faster, and can't be rolled back.

Question 3

Explain PRIMARY KEY and FOREIGN KEY constraints.

Accepted Answer

PRIMARY KEY uniquely identifies each record in a table. FOREIGN KEY establishes relationships between tables by referencing a PRIMARY KEY in another table.

Question 4

What is the difference between INNER and LEFT JOIN?

Accepted Answer

INNER JOIN returns matching records from both tables. LEFT JOIN returns all records from the left table and matching records from the right table.

Question 5

What are SQL aggregate functions?

Accepted Answer

Typical aggregate functions include COUNT, SUM, AVG, MAX, and MIN. They perform calculations on multiple rows and return a single value.

Question 6

Explain the WHERE vs HAVING clause.

Accepted Answer

WHERE filters individual rows before grouping. HAVING filters groups after GROUP BY clause. HAVING works with aggregate functions, WHERE doesn't.

Question 7

What are SQL indexes and their importance?

Accepted Answer

Indexes improve query performance by providing faster data retrieval. They create data structure that speeds up searching and sorting operations.

Question 8

How to remove duplicates in SQL?

Accepted Answer

Use DISTINCT keyword in SELECT statement or GROUP BY clause. For removing duplicate rows, use the DELETE with ROW_NUMBER() function.

Question 9

What is SQL normalization?

Accepted Answer

Normalization organizes database tables to minimize redundancy. Standard forms are 1NF, 2NF, and 3NF, each addressing specific data integrity issues.

Question 10

Explain SQL subqueries.

Accepted Answer

Subqueries (nested queries) are queries within another query. They can be used in SELECT, FROM, and WHERE clauses to perform complex operations.

Question 11

What's the difference between UNION and UNION ALL?

Accepted Answer

UNION combines results and removes duplicates. UNION ALL combines results, including duplicates. UNION ALL performs better as it skips duplicate removal.

Question 12

How to handle NULL values in SQL?

Accepted Answer

Use IS NULL or IS NOT NULL in WHERE clause. COALESCE or ISNULL functions replace NULL with specified values.

Career guidance

sql

Top 15 SQL Interview Questions for Your Next Interview

Choose a topic

Looking for something specific?