Top 25 sql interview questions and answers 2024

Basic SQL Questions:

What is SQL?
- SQL (Structured Query Language) is a standard language for managing relational databases and performing various operations such as querying data, updating data, and defining and modifying the structure of databases.
What are the types of SQL statements?
- SQL statements can be categorized into:
  - Data Query Language (DQL): SELECT
  - Data Definition Language (DDL): CREATE, ALTER, DROP, TRUNCATE
  - Data Manipulation Language (DML): INSERT, UPDATE, DELETE
  - Transaction Control Language (TCL): COMMIT, ROLLBACK, SAVEPOINT
  - Data Control Language (DCL): GRANT, REVOKE
Explain the difference between SQL and MySQL.
- SQL is a language used to operate databases, while MySQL is one of several database management systems that use SQL as its standard language.
What is a primary key?
- A primary key is a unique identifier for a record within a database table. It ensures each row in a table is uniquely identifiable and cannot have NULL values.
What is a foreign key?
- A foreign key is a column or a set of columns in one table that refers to the primary key of another table. It establishes and enforces a link between data in two tables.
What is normalization?
- Normalization is the process of organizing data in a database to reduce redundancy and improve data integrity. It involves dividing large tables into smaller, related tables and defining relationships between them.
What are the different types of joins in SQL?
- SQL joins are used to combine rows from two or more tables based on related columns. Types of joins include INNER JOIN, LEFT JOIN (or LEFT OUTER JOIN), RIGHT JOIN (or RIGHT OUTER JOIN), and FULL JOIN (or FULL OUTER JOIN).
What is the difference between INNER JOIN and OUTER JOIN?
- INNER JOIN returns only the rows that have matching values in both tables. OUTER JOIN returns all rows from both tables, along with matching rows from the tables where available.
What is a subquery?
- A subquery is a query nested within another SQL query. It can be used to retrieve data that will be used by the main query as a condition or filter.
What is the difference between DELETE and TRUNCATE?
- DELETE is a DML command used to remove rows from a table based on a condition, and it can be rolled back. TRUNCATE is a DDL command used to remove all rows from a table, and it cannot be rolled back.

Intermediate SQL Questions:

What is the difference between CHAR and VARCHAR data types?
- CHAR is a fixed-length data type that stores characters with a defined length. VARCHAR is a variable-length data type that stores characters with a maximum length.
Explain the GROUP BY clause.
- GROUP BY clause is used to group rows that have the same values into summary rows, and it is often used with aggregate functions like COUNT, SUM, AVG, etc.
What are aggregate functions in SQL?
- Aggregate functions perform calculations on a set of values and return a single value. Examples include COUNT, SUM, AVG, MIN, and MAX.
Explain the HAVING clause.
- HAVING clause is used in combination with the GROUP BY clause to filter records returned by aggregate functions based on a specified condition.
How do you sort records in SQL?
- Records can be sorted using the ORDER BY clause, which sorts the result set in ascending (default) or descending order based on specified columns.
What is a view in SQL?
- A view is a virtual table derived from one or more tables. It represents the result set of a stored query and can be used like a regular table in SQL queries.
Explain the difference between UNION and UNION ALL.
- UNION combines the results of two or more SELECT statements and removes duplicate rows, while UNION ALL combines the results of two or more SELECT statements including duplicates.
What is a stored procedure?
- A stored procedure is a prepared SQL code that can be saved and reused. It allows you to perform multiple SQL statements in a single call and can accept parameters.
Explain ACID properties in the context of SQL transactions.
- ACID (Atomicity, Consistency, Isolation, Durability) properties are characteristics of transactions that ensure they are reliable and maintain data integrity in multi-user environments.
What is a trigger in SQL?
- A trigger is a special type of stored procedure that automatically executes in response to specific events on a table, such as INSERT, UPDATE, or DELETE operations.

Advanced SQL Questions:

What are window functions in SQL?
- Window functions perform calculations across a set of rows related to the current row and are used with the OVER() clause.
Explain the difference between OLTP and OLAP.
- OLTP (Online Transaction Processing) systems are optimized for transactional processing and real-time data entry. OLAP (Online Analytical Processing) systems are optimized for querying and analyzing large datasets for business intelligence and reporting.
How do you optimize SQL queries?
- SQL queries can be optimized by using indexes, minimizing the use of SELECT *, optimizing WHERE and JOIN clauses, and avoiding nested queries where possible.
What is the difference between a clustered and non-clustered index?
- A clustered index determines the physical order of data in a table and is used for fast retrieval of rows. A non-clustered index is a separate structure that stores a sorted order of rows and is used for faster retrieval of specific rows.
Explain the concept of database transactions.
- A database transaction is a logical unit of work that consists of one or more SQL statements executed as a single unit. Transactions ensure data consistency by allowing operations to either succeed entirely or be rolled back entirely in case of failure.

For More Interview Tips and Questions : Click Here

For Python Interview Questions and answers: Click Here

For Job Alerts : Click Here

For any doubts Ping me : Click Insta