Preparing for an SQL interview?

Whether you are a fresher, SQL developer, software developer, data analyst, QA engineer, business analyst, or experienced database professional, SQL is an important skill to prepare.

This guide covers 75+ SQL interview questions and answers, starting with basic SQL concepts and moving toward joins, aggregate functions, subqueries, CTEs, window functions, indexes, transactions, normalization, and practical SQL coding problems.

The goal is not just to memorize answers. You should understand why a query works, how SQL processes data, and when to use each SQL feature.

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SQL Interview Questions

Table of Contents

1. What is SQL?
2. Why is SQL important in interviews?
3. Basic SQL Interview Questions
4. SQL Commands and Constraints
5. SQL JOIN Interview Questions
6. GROUP BY, HAVING and Aggregate Functions
7. Subquery and CTE Questions
8. SQL Window Function Questions
9. Practical SQL Query Interview Questions
10. Database Design and Normalization
11. Index and Performance Questions
12. Transactions and ACID Questions
13. Advanced SQL Interview Questions
14. Tricky SQL Interview Questions
15. SQL Interview Questions for Freshers
16. SQL Interview Questions for Experienced Candidates
17. SQL Interview Preparation Tips
18. Frequently Asked Questions

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1. What Is SQL?

SQL (Structured Query Language) is a language used to communicate with relational databases.

SQL can be used to:

• Create databases and tables
• Insert data
• Retrieve data
• Update data
• Delete data
• Filter records
• Sort data
• Join multiple tables
• Group and summarize data
• Create views
• Manage transactions
• Analyze data

For example:

SELECT name, salary
FROM employees
WHERE salary > 50000;

This query returns employees whose salary is greater than 50,000.

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2. Why Is SQL Important in Interviews?

SQL interviews usually test two different abilities:

Conceptual knowledge

For example:

• What is a primary key?
• What is normalization?
• What is an index?
• What is a JOIN?
• What is a transaction?

Practical problem-solving

For example:

• Find the second-highest salary.
• Find duplicate records.
• Find employees who do not belong to any department.
• Find the highest-paid employee in each department.
• Calculate a running total.
• Find the top three employees in each department.

Modern SQL interview preparation increasingly includes window functions, CTEs, subqueries, JOINs and practical query-writing, rather than only definitions.

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Part 1: Basic SQL Interview Questions

3. What is a Database?

A database is an organized collection of data that can be stored, accessed, managed and updated electronically.

Examples include:

• MySQL
• PostgreSQL
• Microsoft SQL Server
• Oracle Database
• SQLite

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4. What is a DBMS?

DBMS stands for Database Management System.

It is software used to create, manage and interact with databases.

Examples:

• MySQL
• Oracle
• PostgreSQL
• Microsoft SQL Server

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5. What is an RDBMS?

RDBMS stands for Relational Database Management System.

An RDBMS stores data primarily in tables consisting of rows and columns and provides relationships between tables.

Examples:

• MySQL
• PostgreSQL
• Oracle
• SQL Server

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6. What is a Table in SQL?

A table stores data in rows and columns.

Example:

id	name	salary
1	Amit	45000
2	Priya	60000
3	Rahul	52000

Here:

• id, name, and salary are columns.
• Each employee record is a row.

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7. What is a Primary Key?

A primary key uniquely identifies each row in a table.

Example:

CREATE TABLE employees (
    employee_id INT PRIMARY KEY,
    name VARCHAR(100),
    salary DECIMAL(10,2)
);

employee_id uniquely identifies each employee.

A primary key:

• Must be unique
• Cannot contain NULL
• Identifies a record

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8. What is a Foreign Key?

A foreign key creates a relationship between tables.

Example:

CREATE TABLE departments (
    department_id INT PRIMARY KEY,
    department_name VARCHAR(100)
);

Another table can reference it:

CREATE TABLE employees (
    employee_id INT PRIMARY KEY,
    name VARCHAR(100),
    department_id INT,
    FOREIGN KEY (department_id)
        REFERENCES departments(department_id)
);

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9. What is a UNIQUE Constraint?

A UNIQUE constraint prevents duplicate values in a column or group of columns.

Example:

CREATE TABLE users (
    id INT PRIMARY KEY,
    email VARCHAR(150) UNIQUE
);

Two users cannot have the same email value.

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10. What is a NOT NULL Constraint?

NOT NULL prevents a column from containing NULL.

CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(100) NOT NULL
);

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Part 2: SQL Commands

11. What are DDL, DML, DQL, DCL and TCL?

SQL commands are commonly grouped into several categories.

DDL – Data Definition Language

Used to define database structures.

Examples:

CREATE
ALTER
DROP
TRUNCATE

DML – Data Manipulation Language

Used to modify data.

INSERT
UPDATE
DELETE

DQL – Data Query Language

Primarily used to retrieve data.

SELECT

DCL – Data Control Language

Used for permissions.

GRANT
REVOKE

TCL – Transaction Control Language

Used to manage transactions.

COMMIT
ROLLBACK
SAVEPOINT

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Part 3: DELETE vs TRUNCATE vs DROP

12. What is the difference between DELETE, TRUNCATE and DROP?

Command	Purpose
DELETE	Removes rows
TRUNCATE	Removes all rows from a table
DROP	Removes the table itself

DELETE

DELETE FROM employees
WHERE department_id = 5;

You can use a WHERE condition with DELETE.

TRUNCATE

TRUNCATE TABLE employees;

It removes all rows from the table.

DROP

DROP TABLE employees;

This removes the table definition and its data.

Interview tip: The exact transaction, logging, identity-reset and trigger behavior of TRUNCATE can vary by database system, so answer according to the SQL engine being discussed.

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Part 4: SELECT, WHERE and ORDER BY

13. How do you retrieve all records from a table?

SELECT *
FROM employees;

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14. How do you select specific columns?

SELECT name, salary
FROM employees;

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15. How do you filter records?

Use the WHERE clause.

SELECT *
FROM employees
WHERE salary > 50000;

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16. How do you sort records?

Use ORDER BY.

SELECT *
FROM employees
ORDER BY salary DESC;

ASC means ascending.

DESC means descending.

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Part 5: WHERE vs HAVING

17. What is the difference between WHERE and HAVING?

This is one of the most common SQL interview questions.

WHERE

Filters individual rows before grouping.

SELECT *
FROM employees
WHERE salary > 50000;

HAVING

Filters groups after aggregation.

SELECT department_id, AVG(salary) AS avg_salary
FROM employees
GROUP BY department_id
HAVING AVG(salary) > 50000;

Easy way to remember

WHERE → filters rows

HAVING → filters groups

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Part 6: GROUP BY and Aggregate Functions

18. What is GROUP BY?

GROUP BY combines rows having the same value so that aggregate calculations can be performed.

Example:

SELECT department_id, COUNT(*) AS employee_count
FROM employees
GROUP BY department_id;

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19. What are aggregate functions?

Common aggregate functions include:

• COUNT()
• SUM()
• AVG()
• MIN()
• MAX()

Example:

SELECT
    COUNT(*) AS total_employees,
    AVG(salary) AS average_salary,
    MAX(salary) AS highest_salary,
    MIN(salary) AS lowest_salary
FROM employees;

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Part 7: SQL JOIN Interview Questions

JOINs are among the most important SQL interview topics because they test whether you understand relationships between tables.

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20. What is an INNER JOIN?

An INNER JOIN returns rows where matching records exist in both tables.

SELECT
    e.name,
    d.department_name
FROM employees e
INNER JOIN departments d
    ON e.department_id = d.department_id;

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21. What is a LEFT JOIN?

A LEFT JOIN returns all rows from the left table and matching rows from the right table.

If no match exists, columns from the right table generally contain NULL.

SELECT
    e.name,
    d.department_name
FROM employees e
LEFT JOIN departments d
    ON e.department_id = d.department_id;

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22. What is a RIGHT JOIN?

A RIGHT JOIN returns all rows from the right table and matching rows from the left table.

SELECT
    e.name,
    d.department_name
FROM employees e
RIGHT JOIN departments d
    ON e.department_id = d.department_id;

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23. What is a FULL OUTER JOIN?

A FULL OUTER JOIN returns matching rows plus unmatched rows from both tables.

Conceptually:

SELECT *
FROM employees e
FULL OUTER JOIN departments d
    ON e.department_id = d.department_id;

Support for FULL OUTER JOIN differs between database systems, so always consider the SQL dialect.

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24. What is a CROSS JOIN?

A CROSS JOIN produces the Cartesian product of two tables.

If table A contains 3 rows and table B contains 4 rows, the result can contain:

3 × 4 = 12 rows

Example:

SELECT *
FROM colors
CROSS JOIN sizes;

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25. What is a SELF JOIN?

A self join joins a table to itself.

It is useful for hierarchical data such as employees and managers.

SELECT
    e.name AS employee,
    m.name AS manager
FROM employees e
LEFT JOIN employees m
    ON e.manager_id = m.employee_id;

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Part 8: UNION vs UNION ALL

26. What is the difference between UNION and UNION ALL?

UNION

Combines result sets and removes duplicate rows.

SELECT city FROM customers
UNION
SELECT city FROM suppliers;

UNION ALL

Combines result sets without removing duplicates.

SELECT city FROM customers
UNION ALL
SELECT city FROM suppliers;

If duplicates do not need to be removed, UNION ALL can avoid the extra duplicate-elimination work.

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Part 9: NULL in SQL

27. What is NULL?

NULL represents an unknown, missing or unavailable value.

It is not the same as:

• 0
• Empty string
• FALSE

To test for NULL, use:

WHERE department_id IS NULL;

Not:

WHERE department_id = NULL;

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28. What is COALESCE?

COALESCE() returns the first non-NULL expression.

Example:

SELECT
    name,
    COALESCE(phone, 'Not Available') AS phone
FROM customers;

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Part 10: Subquery Interview Questions

29. What is a Subquery?

A subquery is a query inside another SQL query.

Example:

SELECT name, salary
FROM employees
WHERE salary > (
    SELECT AVG(salary)
    FROM employees
);

This returns employees whose salary is greater than the average salary.

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30. What is a Correlated Subquery?

A correlated subquery references a value from the outer query.

Example:

SELECT e1.name, e1.salary, e1.department_id
FROM employees e1
WHERE e1.salary > (
    SELECT AVG(e2.salary)
    FROM employees e2
    WHERE e2.department_id = e1.department_id
);

Here, the inner query depends on the current row from the outer query.

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31. What is the difference between IN and EXISTS?

IN compares a value against a set of values.

SELECT *
FROM customers
WHERE customer_id IN (
    SELECT customer_id
    FROM orders
);

EXISTS checks whether the subquery returns at least one matching row.

SELECT *
FROM customers c
WHERE EXISTS (
    SELECT 1
    FROM orders o
    WHERE o.customer_id = c.customer_id
);

The best choice depends on the query, data distribution, optimizer and database engine.

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Part 11: CTE Interview Questions

32. What is a CTE?

CTE stands for Common Table Expression.

It is defined using WITH.

Example:

WITH high_salary AS (
    SELECT *
    FROM employees
    WHERE salary > 70000
)
SELECT *
FROM high_salary;

CTEs can make complex queries easier to read and maintain.

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33. What is a Recursive CTE?

A recursive CTE refers to itself and is useful for hierarchical data.

Common examples include:

• Employee-manager hierarchies
• Folder structures
• Organizational trees
• Category hierarchies

Example:

WITH RECURSIVE employee_tree AS (
    SELECT employee_id, name, manager_id
    FROM employees
    WHERE manager_id IS NULL

    UNION ALL

    SELECT e.employee_id, e.name, e.manager_id
    FROM employees e
    JOIN employee_tree t
        ON e.manager_id = t.employee_id
)
SELECT *
FROM employee_tree;

The exact syntax varies by database.

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Part 12: Window Function Interview Questions

Window functions are especially useful for ranking, running totals, comparisons with previous/next rows, and top-N-per-group problems.

34. What is a Window Function?

A window function calculates a value across related rows without collapsing the rows into one result per group.

Example:

SELECT
    name,
    department_id,
    salary,
    AVG(salary) OVER (
        PARTITION BY department_id
    ) AS department_avg
FROM employees;

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35. What is ROW_NUMBER()?

ROW_NUMBER() assigns a unique sequential number to rows.

SELECT
    name,
    salary,
    ROW_NUMBER() OVER (
        ORDER BY salary DESC
    ) AS row_num
FROM employees;

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36. What is RANK()?

RANK() gives the same rank to tied values and leaves gaps after ties.

For salaries:

Salary	RANK
100000	1
100000	1
90000	3

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37. What is DENSE_RANK()?

DENSE_RANK() also gives the same rank to ties, but does not leave gaps.

Salary	DENSE_RANK
100000	1
100000	1
90000	2

Knowing the difference between ROW_NUMBER(), RANK() and DENSE_RANK() is important for interview problems involving ties and ranking.

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38. What is the difference between ROW_NUMBER, RANK and DENSE_RANK?

Function	Handles ties	Skips rank
ROW_NUMBER	No shared rank	No
RANK	Same rank	Yes
DENSE_RANK	Same rank	No

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Part 13: Practical SQL Coding Interview Questions

Now let’s move from theory to the type of SQL problems you may be asked to solve during an interview.

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39. How do you find the second-highest salary?

One approach is:

SELECT MAX(salary) AS second_highest
FROM employees
WHERE salary < (
    SELECT MAX(salary)
    FROM employees
);

This treats duplicate salary values as one salary level.

Another approach is:

SELECT salary
FROM (
    SELECT
        salary,
        DENSE_RANK() OVER (
            ORDER BY salary DESC
        ) AS rnk
    FROM employees
) x
WHERE rnk = 2;

The window-function solution is particularly useful when the interviewer asks you to explain how ties should be handled.

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40. How do you find the highest salary in each department?

SELECT
    department_id,
    MAX(salary) AS highest_salary
FROM employees
GROUP BY department_id;

If the interviewer also wants the employee’s name, a window function can be useful:

WITH ranked AS (
    SELECT
        name,
        department_id,
        salary,
        RANK() OVER (
            PARTITION BY department_id
            ORDER BY salary DESC
        ) AS rnk
    FROM employees
)
SELECT name, department_id, salary
FROM ranked
WHERE rnk = 1;

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41. How do you find the top 3 employees in each department?

WITH ranked AS (
    SELECT
        employee_id,
        name,
        department_id,
        salary,
        ROW_NUMBER() OVER (
            PARTITION BY department_id
            ORDER BY salary DESC
        ) AS rn
    FROM employees
)
SELECT *
FROM ranked
WHERE rn <= 3;

If ties should all be included, consider RANK() or DENSE_RANK() instead.

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42. How do you find duplicate records?

Suppose duplicate email addresses are stored in a users table.

SELECT
    email,
    COUNT(*) AS total
FROM users
GROUP BY email
HAVING COUNT(*) > 1;

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43. How do you find employees who do not have a department?

Using a LEFT JOIN:

SELECT e.*
FROM employees e
LEFT JOIN departments d
    ON e.department_id = d.department_id
WHERE d.department_id IS NULL;

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44. How do you find customers who have never placed an order?

SELECT c.*
FROM customers c
LEFT JOIN orders o
    ON c.customer_id = o.customer_id
WHERE o.customer_id IS NULL;

An equivalent NOT EXISTS approach is:

SELECT c.*
FROM customers c
WHERE NOT EXISTS (
    SELECT 1
    FROM orders o
    WHERE o.customer_id = c.customer_id
);

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45. How do you calculate the total salary by department?

SELECT
    department_id,
    SUM(salary) AS total_salary
FROM employees
GROUP BY department_id;

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46. How do you calculate average salary by department?

SELECT
    department_id,
    AVG(salary) AS average_salary
FROM employees
GROUP BY department_id;

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47. How do you find employees earning more than the company average?

SELECT name, salary
FROM employees
WHERE salary > (
    SELECT AVG(salary)
    FROM employees
);

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48. How do you find employees earning more than their department average?

SELECT
    name,
    department_id,
    salary
FROM employees e
WHERE salary > (
    SELECT AVG(e2.salary)
    FROM employees e2
    WHERE e2.department_id = e.department_id
);

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Part 14: Running Total

49. How do you calculate a running total?

Suppose you have:

sale_date
amount

You can use:

SELECT
    sale_date,
    amount,
    SUM(amount) OVER (
        ORDER BY sale_date
    ) AS running_total
FROM sales;

This is a common example of why window functions are useful: the rows remain visible while the cumulative calculation is added.

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Part 15: LAG and LEAD

50. What is LAG()?

LAG() retrieves a value from a previous row.

Example:

SELECT
    sale_date,
    amount,
    LAG(amount) OVER (
        ORDER BY sale_date
    ) AS previous_amount
FROM sales;

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51. What is LEAD()?

LEAD() retrieves a value from a following row.

SELECT
    sale_date,
    amount,
    LEAD(amount) OVER (
        ORDER BY sale_date
    ) AS next_amount
FROM sales;

These functions are useful for:

• Month-over-month comparisons
• Previous transaction analysis
• Change detection
• Time-series analysis

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Part 16: Database Normalization

52. What is Normalization?

Normalization is a database-design process used to organize data and reduce unnecessary duplication and update anomalies.

Common normal forms include:

• 1NF
• 2NF
• 3NF
• BCNF

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53. What is First Normal Form (1NF)?

A table generally satisfies 1NF when each field contains atomic values rather than repeating groups or multiple values packed into a single field.

Bad example:

Student	Subjects
Rahul	Maths, Science, English

A normalized design could store subjects as separate rows.

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54. What is Second Normal Form (2NF)?

2NF builds on 1NF and removes partial dependency on part of a composite key.

It is mainly relevant when a table has a composite primary key.

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55. What is Third Normal Form (3NF)?

3NF builds on 2NF and aims to remove transitive dependencies.

In simple terms, non-key attributes should depend on the key rather than on another non-key attribute.

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Part 17: SQL Index Interview Questions

56. What is an Index?

An index is a database structure that can help the database locate rows more efficiently.

Example:

CREATE INDEX idx_employee_email
ON employees(email);

Indexes can improve read performance for suitable queries, but they also consume storage and can add overhead to data modifications.

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57. What are the disadvantages of indexes?

Indexes can:

• Consume additional storage
• Increase the cost of INSERT
• Increase the cost of UPDATE
• Increase the cost of DELETE
• Require maintenance
• Be ineffective for some query patterns

Therefore, adding an index to every column is not a good general strategy.

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58. What is a composite index?

A composite index contains more than one column.

CREATE INDEX idx_employee_dept_salary
ON employees(department_id, salary);

Column order matters because the database can use the index differently depending on the query and database optimizer.

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Part 18: Transactions and ACID

59. What is a Transaction?

A transaction is a logical unit of database work.

For example:

BEGIN;

UPDATE accounts
SET balance = balance - 1000
WHERE account_id = 1;

UPDATE accounts
SET balance = balance + 1000
WHERE account_id = 2;

COMMIT;

Both operations are intended to be part of the same transaction.

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60. What is ACID?

ACID stands for:

Atomicity

A transaction is treated as a unit.

Consistency

The database should remain in a valid state according to its rules.

Isolation

Concurrent transactions should behave according to the database’s isolation guarantees.

Durability

Committed changes should persist despite subsequent failures, subject to the database system’s guarantees.

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61. What is COMMIT?

COMMIT permanently saves a transaction.

COMMIT;

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62. What is ROLLBACK?

ROLLBACK reverses changes that are still eligible to be rolled back within the current transaction.

ROLLBACK;

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Part 19: Views

63. What is a View?

A view is a named query that can be queried like a table.

Example:

CREATE VIEW high_salary_employees AS
SELECT name, salary
FROM employees
WHERE salary > 70000;

Then:

SELECT *
FROM high_salary_employees;

Views can simplify access to commonly used queries and can also help expose only selected columns or rows.

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Part 20: Stored Procedures

64. What is a Stored Procedure?

A stored procedure is a database-side program containing SQL statements and procedural logic supported by the database system.

The exact syntax differs between MySQL, SQL Server, PostgreSQL and Oracle.

Example syntax in a MySQL-style environment:

CREATE PROCEDURE GetEmployees()
BEGIN
    SELECT *
    FROM employees;
END;

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Part 21: Triggers

65. What is a Trigger?

A trigger is database logic that automatically executes in response to specified events such as:

• INSERT
• UPDATE
• DELETE

Example use cases include:

• Audit logging
• Maintaining derived data
• Enforcing certain business rules

Triggers should be designed carefully because they can make data changes less obvious to application developers.

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Part 22: SQL Query Execution Order

66. What is the logical order of SQL query execution?

A simplified logical processing order is:

FROM
JOIN
WHERE
GROUP BY
HAVING
SELECT
DISTINCT
ORDER BY
LIMIT / OFFSET

The exact implementation is optimizer-dependent, but understanding logical processing order helps explain questions such as why a window-function result generally cannot be filtered directly in the same query’s WHERE clause.

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Part 23: Tricky SQL Interview Questions

67. Can WHERE be used with aggregate functions?

Generally, aggregate results such as COUNT() or AVG() are filtered using HAVING after grouping.

Example:

SELECT department_id, COUNT(*) AS total
FROM employees
GROUP BY department_id
HAVING COUNT(*) > 10;

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68. Can we use a column alias in WHERE?

In many SQL systems, a SELECT alias cannot be referenced in the same query block’s WHERE because of the logical processing order.

For example:

SELECT salary * 12 AS annual_salary
FROM employees
WHERE annual_salary > 600000;

may not work depending on the database.

A derived table or CTE can be used instead:

SELECT *
FROM (
    SELECT
        name,
        salary * 12 AS annual_salary
    FROM employees
) x
WHERE annual_salary > 600000;

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69. What happens when NULL is compared using =?

Consider:

SELECT *
FROM employees
WHERE department_id = NULL;

This does not correctly test for NULL.

Use:

WHERE department_id IS NULL;

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70. What is the NOT IN and NULL problem?

This is an important SQL edge case.

Consider:

WHERE employee_id NOT IN (
    SELECT employee_id
    FROM employees_backup
);

If the subquery contains NULL, three-valued SQL logic can produce unexpected results.

For many “does not exist” problems, NOT EXISTS is safer when NULL semantics are relevant:

WHERE NOT EXISTS (
    SELECT 1
    FROM employees_backup b
    WHERE b.employee_id = e.employee_id
);

The important interview point is to understand NULL semantics, rather than blindly choosing one syntax.

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Part 24: More Practical SQL Interview Problems

71. Find the third-highest salary

Using DENSE_RANK():

WITH ranked AS (
    SELECT
        salary,
        DENSE_RANK() OVER (
            ORDER BY salary DESC
        ) AS rnk
    FROM employees
)
SELECT salary
FROM ranked
WHERE rnk = 3;

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72. Find employees who have the same salary

SELECT salary, COUNT(*) AS employee_count
FROM employees
GROUP BY salary
HAVING COUNT(*) > 1;

---

73. Find the number of employees in each department

SELECT
    department_id,
    COUNT(*) AS employee_count
FROM employees
GROUP BY department_id;

---

74. Find departments having more than 5 employees

SELECT
    department_id,
    COUNT(*) AS employee_count
FROM employees
GROUP BY department_id
HAVING COUNT(*) > 5;

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75. Find the employee with the highest salary

SELECT *
FROM employees
WHERE salary = (
    SELECT MAX(salary)
    FROM employees
);

This returns all employees tied at the maximum salary.

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76. Find the latest order for each customer

A window function is one way:

WITH ranked_orders AS (
    SELECT
        order_id,
        customer_id,
        order_date,
        ROW_NUMBER() OVER (
            PARTITION BY customer_id
            ORDER BY order_date DESC
        ) AS rn
    FROM orders
)
SELECT *
FROM ranked_orders
WHERE rn = 1;

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77. Find monthly sales

A database-specific date function may be required.

Conceptually:

SELECT
    YEAR(order_date) AS sales_year,
    MONTH(order_date) AS sales_month,
    SUM(amount) AS total_sales
FROM orders
GROUP BY
    YEAR(order_date),
    MONTH(order_date);

The exact date functions vary between MySQL, SQL Server, PostgreSQL and Oracle.

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SQL Interview Questions for Freshers

If you are a fresher, focus first on these topics:

1. What is SQL?
2. DBMS vs RDBMS
3. Primary key
4. Foreign key
5. Constraints
6. DDL vs DML
7. DELETE vs TRUNCATE vs DROP
8. SELECT
9. WHERE
10. ORDER BY
11. GROUP BY
12. HAVING
13. Aggregate functions
14. INNER JOIN
15. LEFT JOIN
16. UNION vs UNION ALL
17. NULL
18. Subqueries
19. Views
20. Basic SQL queries

After mastering these, move to:

• CTEs
• Window functions
• Indexes
• Transactions
• Query optimization

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SQL Interview Questions for Experienced Candidates

Experienced candidates should be prepared for SQL Interview Questions more than definitions.

Important topics include:

Query optimization

• Index selection
• Execution plans
• Join strategies
• Large-table queries
• Avoiding unnecessary scans

Advanced SQL

• CTEs
• Recursive CTEs
• Window functions
• Correlated subqueries
• Complex joins
• Conditional aggregation

Database design

• Normalization
• Denormalization
• Keys
• Relationships
• Constraints

Transactions

• ACID
• Isolation levels
• Locking
• Deadlocks
• Concurrency

Practical coding

• Top N per group
• Running totals
• Duplicate detection
• Missing records
• Gaps and sequences
• Previous/next row comparisons

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15 SQL Queries You Should Practice Before an Interview

Before your interview, try writing these queries without looking at the solution:

Beginner

1. Select all employees.
2. Find employees earning more than ₹50,000.
3. Sort employees by salary.
4. Count employees.
5. Find average salary.

Intermediate

6. Find employees in each department.
7. Find departments with more than 5 employees.
8. Find the second-highest salary.
9. Find duplicate emails.
10. Find customers without orders.

Advanced

11. Find the highest-paid employee in each department.
12. Find the top 3 employees in each department.
13. Calculate a running total.
14. Find each employee’s previous salary record.
15. Find the latest order for every customer.

These practical patterns cover many of the recurring SQL coding concepts discussed in current SQL Interview Questions -preparation material.

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Common SQL Interview Mistakes / SQL Interview Questions Mistake

Avoid these mistakes during an SQL interview.

Mistake 1: Memorizing queries

Instead of memorizing:

SELECT ...

understand what the query is doing.

Mistake 2: Ignoring NULL

Always consider how NULL affects comparisons, joins and NOT IN.

Mistake 3: Confusing WHERE and HAVING

Remember:

WHERE → rows

HAVING → groups

Mistake 4: Ignoring ties

For ranking problems, ask whether duplicate values should receive the same rank.

Mistake 5: Using SELECT *

In production queries, selecting only the required columns can make queries clearer and may reduce unnecessary data retrieval.

Mistake 6: Adding indexes everywhere

Indexes can improve some reads but add storage and write overhead.

Mistake 7: Not explaining your logic

During a coding interview, explain:

1. What you are trying to find.
2. Which tables you need.
3. How the tables are related.
4. Why you selected a particular JOIN.
5. How NULLs and duplicates are handled.
6. Why your query produces the expected result.

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How to Prepare for an SQL Interview Questions

Step 1: Learn SQL fundamentals

Start with:

SELECT
WHERE
ORDER BY
GROUP BY
HAVING

Step 2: Master JOINs

Practice:

INNER JOIN
LEFT JOIN
RIGHT JOIN
FULL OUTER JOIN
SELF JOIN
CROSS JOIN

Step 3: Practice aggregate functions

Learn:

COUNT()
SUM()
AVG()
MIN()
MAX()

Step 4: Learn subqueries and CTEs

Practice rewriting problems using:

• Subqueries
• CTEs
• JOINs

Step 5: Learn window functions

Focus on:

ROW_NUMBER()
RANK()
DENSE_RANK()
LAG()
LEAD()
SUM() OVER()
AVG() OVER()

Step 6: Solve practical problems

Do not only read SQL.

Write the queries yourself.

Try solving a problem first, then compare your answer with the solution.

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Quick SQL Interview Questions Cheat Sheet

Topic	Remember
WHERE	Filters rows
HAVING	Filters groups
GROUP BY	Groups rows
INNER JOIN	Matching rows
LEFT JOIN	All left + matches
UNION	Combines and removes duplicates
UNION ALL	Combines without duplicate removal
PRIMARY KEY	Unique row identifier
FOREIGN KEY	Table relationship
INDEX	Helps suitable lookups
CTE	Named temporary query expression
ROW_NUMBER	Unique sequence
RANK	Ties + gaps
DENSE_RANK	Ties without gaps
LAG	Previous row
LEAD	Next row
COMMIT	Save transaction
ROLLBACK	Undo eligible transaction changes
NULL	Missing/unknown value

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Frequently Asked Questions About SQL Interview Questions

What are the most common SQL interview questions?

Common areas include JOINs, GROUP BY, HAVING, aggregate functions, subqueries, window functions, indexes, transactions and practical query-writing.

Is SQL difficult for freshers?

The fundamentals can be learned progressively. The most important step is to practice writing queries rather than only reading SQL syntax.

Which SQL topics should a fresher learn first?

Start with:

• SELECT
• WHERE
• ORDER BY
• GROUP BY
• HAVING
• Aggregate functions
• JOINs
• Subqueries

Then move to CTEs and window functions.

What SQL queries are commonly asked in interviews?

Popular practical problems include:

• Second-highest salary
• Highest salary per department
• Duplicate records
• Employees without departments
• Customers without orders
• Top N records per group
• Running totals
• Latest record per customer

Should I learn MySQL or PostgreSQL for interviews?

Either can be a good choice. However, SQL syntax and features differ between database systems, so check which database your target role uses.

Are SQL JOINs important for interviews?

Yes. JOIN problems test whether you understand how information stored across multiple tables can be combined.

Are window functions important?

They are particularly useful for ranking, running calculations and comparing rows. They are also common in practical SQL interview questions.

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Final SQL Interview Questions Preparation Checklist

Before attending your SQL interview, make sure you can confidently explain and write:

• SELECT
• WHERE
• ORDER BY
• GROUP BY
• HAVING
• Aggregate functions
• Primary key
• Foreign key
• Constraints
• INNER JOIN
• LEFT JOIN
• RIGHT JOIN
• FULL OUTER JOIN
• SELF JOIN
• UNION
• UNION ALL
• NULL
• Subqueries
• Correlated subqueries
• CTEs
• Window functions
• ROW_NUMBER
• RANK
• DENSE_RANK
• LAG
• LEAD
• Indexes
• Views
• Transactions
• ACID
• Normalization
• Query optimization
• Second-highest salary
• Top N per group
• Duplicate records
• Running totals
• Customers without orders

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Conclusion

SQL interviews are not only about remembering SQL commands. Interviewers often want to see whether you can understand a data problem, choose the appropriate SQL technique, write a correct query, and explain your reasoning.

Start with SQL fundamentals, become comfortable with JOINs and aggregation, and then practice subqueries, CTEs and window functions.

Most importantly, write SQL queries yourself. Reading a solution is useful, but solving the problem without looking at the answer is what builds interview confidence.

Bookmark this SQL Interview Questions and Answers guide and use it as a revision checklist before your next technical interview.

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