Question 1

What is SQL?

Accepted Answer

SQL stands for Structured Query Language. It is a standard programming language used to manage and manipulate relational databases. SQL is used for tasks such as querying data, updating data, and creating and modifying database schemas.

Question 2

What are the types of SQL statements?

Accepted Answer

There are mainly three types of SQL statements:
DDL (Data Definition Language): Used to define the database structure, such as creating, altering, and deleting tables.
DML (Data Manipulation Language): Used to manipulate the data stored in the database, such as inserting, updating, and deleting records.
DQL (Data Query Language): Used to query the data from the database, primarily using the SELECT statement.

Question 3

Explain the difference between INNER JOIN and LEFT JOIN.

Accepted Answer

INNER JOIN: Returns only the rows where there is a match in both tables.
LEFT JOIN (or LEFT OUTER JOIN): Returns all rows from the left table and the matched rows from the right table. If there is no match, NULL values are returned for columns from the right table.

Question 4

What is a primary key?

Accepted Answer

A primary key is a column or a set of columns that uniquely identifies each row in a table. It must contain unique values, and it cannot have NULL values.

Question 5

Explain the ACID properties in the context of databases.

Accepted Answer

ACID stands for Atomicity, Consistency, Isolation, and Durability. These properties ensure the reliability of transactions in a database:
Atomicity: Transactions are treated as a single, indivisible unit of work. Either all changes are committed, or none are.
Consistency: Transactions bring the database from one valid state to another. The database must satisfy predefined integrity constraints.
Isolation: Each transaction is executed in isolation from other transactions. Intermediate states of a transaction are not visible to other transactions.
Durability: Once a transaction is committed, its changes are permanent and survive subsequent failures.

tricky sql queries for interview

SQL Interview Questions CHEAT SHEET (2025)