Top 10 most asked SQL interview questions along with their answers
Tags: sql, database, computerscience, career
Published: 2024-05-06 12:17:32


**1. What is SQL?**
   **Answer:** SQL stands for Structured Query Language. It is a domain-specific language used for managing and manipulating relational databases. SQL is used for querying data, updating data, defining schema, and managing permissions within a database system.

**2. What are the different types of SQL commands?**
   **Answer:** SQL commands are broadly classified into four categories:
   - Data Definition Language (DDL): Used for defining and modifying database structure (e.g., CREATE, ALTER, DROP).
   - Data Manipulation Language (DML): Used for managing data within tables (e.g., INSERT, UPDATE, DELETE).
   - Data Query Language (DQL): Used for querying data from tables (e.g., SELECT).
   - Data Control Language (DCL): Used for managing permissions and access control (e.g., GRANT, REVOKE).

**3. Explain the differences between SQL JOINs: INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN.**
   **Answer:** 
   - INNER JOIN: Returns records that have matching values in both tables.
   - LEFT JOIN (or LEFT OUTER JOIN): Returns all records from the left table and matching records from the right table.
   - RIGHT JOIN (or RIGHT OUTER JOIN): Returns all records from the right table and matching records from the left table.
   - FULL JOIN (or FULL OUTER JOIN): Returns all records when there is a match in either the left or right table.

**4. What is the difference between SQL WHERE and HAVING clauses?**
   **Answer:** 
   - WHERE clause is used to filter records before grouping or aggregating data.
   - HAVING clause is used to filter records after grouping or aggregating data.

**5. What is a primary key and a foreign key in SQL?**
   **Answer:** 
   - Primary Key: A primary key is a unique identifier for each record in a table. It ensures that each row in a table is uniquely identified.
   - Foreign Key: A foreign key is a field in one table that refers to the primary key in another table. It establishes a relationship between two tables.

**6. What is the difference between DELETE and TRUNCATE commands?**
   **Answer:** 
   - DELETE command is used to remove one or more records from a table based on a condition.
   - TRUNCATE command is used to remove all records from a table, resetting the table to its original state.

**7. Explain the ACID properties of a transaction in SQL.**
   **Answer:** 
   - ACID stands for Atomicity, Consistency, Isolation, and Durability.
   - Atomicity ensures that either all the operations in a transaction are completed successfully or none of them are.
   - Consistency ensures that the database remains in a consistent state before and after the transaction.
   - Isolation ensures that the operations in one transaction are isolated from other concurrent transactions.
   - Durability ensures that once a transaction is committed, its changes are permanently saved even in the event of a system failure.

**8. What is a subquery in SQL?**
   **Answer:** 
   A subquery is a query nested within another query. It can be used to retrieve data from one or more tables based on the results of the outer query.

**9. Explain the difference between UNION and UNION ALL operators in SQL.**
   **Answer:** 
   - UNION operator is used to combine the result sets of two or more SELECT statements and removes duplicate rows.
   - UNION ALL operator is used to combine the result sets of two or more SELECT statements, including duplicate rows.

**10. What is normalization in SQL?**
   **Answer:** 
   Normalization is the process of organizing data in a database to reduce redundancy and dependency. It involves breaking down large tables into smaller tables and defining relationships between them to minimize data redundancy and ensure data integrity.