SQL interview Questions & Answers
Complete Roadmap to Learn SQL (Structured Query Language)
Week 1: SQL Basics
- What is SQL and how databases work
- Install MySQL Workbench or PostgreSQL
- Learn SELECT, FROM, WHERE
- Filtering data with conditions
- Practice basic queries
Example: Fetch all employees, filter salary > 50k
Week 2: Sorting and Aggregation
- ORDER BY (sorting data)
- Aggregate functions: COUNT, SUM, AVG, MIN, MAX
- GROUP BY concept
- HAVING clause
Example: Department-wise average salary
Week 3: Joins (Most Important )
- INNER JOIN
- LEFT JOIN, RIGHT JOIN
- FULL JOIN
- Self Join
Example: Combine employees and departments tables
Week 4: Advanced Filtering
- IN, BETWEEN, LIKE
- Wildcards (% , _)
- NULL handling (IS NULL, IS NOT NULL)
- CASE statements
Example: Categorize customers based on spending
Week 5: Subqueries
- Nested queries
- Correlated subqueries
- Using subqueries in SELECT, WHERE
Example: Find employees earning above average salary
Week 6: Window Functions (High Value )
- OVER() clause
- ROW_NUMBER(), RANK(), DENSE_RANK()
- PARTITION BY
Example: Rank employees by salary within each department
Week 7: CTE & Views
- Common Table Expressions (WITH)
- Temporary vs permanent views
- Simplify complex queries
Example: Multi-step data transformation
Week 8: Data Modification
- INSERT, UPDATE, DELETE
- TRUNCATE vs DELETE
- Constraints (PRIMARY KEY, FOREIGN KEY)
Example: Update employee salary
Week 9: Indexing & Performance
- What are indexes
- Query optimization basics
- EXPLAIN keyword
Example: Speed up large table queries
Week 10: Working with Real Data
- Import CSV data
- Data cleaning in SQL
- Handling duplicates
- Basic transformations
Example: Clean messy sales dataset
Week 11: Mini Projects
- Write complex queries
- Solve real-world case studies
- Focus on business logic
Examples: Sales dashboard queries, Customer segmentation
Week 12: Final Preparation
- Revise all concepts
- Practice interview questions
- Solve SQL challenges on LeetCode / HackerRank
- Mock interviews
Daily Rule for You
- Practice SQL 60 minutes daily
- Solve 5 queries daily
- Revise previous queries weekly
Pro Tip
- Focus more on JOINS + WINDOW FUNCTIONS
- Practice real datasets, not just theory
- Think in terms of “business questions”
Core SQL Interview Questions With Answers
1 What is SQL
• SQL stands for Structured Query Language
• You use it to read and manage data in relational databases
• Used in MySQL, PostgreSQL, SQL Server, Oracle
2 What is an RDBMS
• Relational Database Management System
• Stores data in tables with rows and columns
• Uses keys to link tables
• Example. Customer table linked to Orders table using customer_id
3 What is a table
• Structured storage for data
• Rows are records
• Columns are attributes
• Example. One row equals one customer
4 What is a primary key
• Uniquely identifies each row
• Cannot be NULL
• No duplicate values
• Example. user_id in users table
5 What is a foreign key
• Links one table to another
• Refers to a primary key in another table
• Allows duplicate values
• Example. user_id in orders table
6 Difference between primary key and foreign key
• Primary key ensures uniqueness
• Foreign key ensures relationship
• One table can have one primary key
• One table can have multiple foreign keys
7 What is NULL
• Represents missing or unknown value
• Not equal to zero or empty string
• Use IS NULL or IS NOT NULL to check
8 What are constraints
• Rules applied on columns
• Maintain data quality
• Common constraints
– NOT NULL
– UNIQUE
– PRIMARY KEY
– FOREIGN KEY
– CHECK
9 What are data types
• Define type of data stored
• Common types
– INT for numbers
– VARCHAR for text
– DATE for dates
– FLOAT or DECIMAL for decimals
10 Interview tip you must remember
• Always explain with a small example
• Speak logic before syntax
• Keep answers short and direct
11. What is SELECT
- Retrieves specific data from tables
- Choose columns with * for all
- Example: SELECT name, age FROM users;
12. What does WHERE do
- Filters rows based on conditions
- Applied after FROM clause
- Example: SELECT * FROM users WHERE age > 25 [1]
13. What is ORDER BY
- Sorts result set by column(s)
- ASC (default) or DESC
- Example: SELECT * FROM users ORDER BY age DESC;
14. What is GROUP BY
- Groups rows with same values
- Used with aggregate functions like COUNT, SUM
- Example: SELECT department, COUNT(*) FROM employees GROUP BY department [2]
15. Difference between WHERE and HAVING
- WHERE filters rows before grouping
- HAVING filters groups after GROUP BY
- WHERE can't use aggregates; HAVING can
16. What is a JOIN
- Combines rows from two or more tables
- Based on related columns
- INNER JOIN returns matching rows only
17. Types of JOINs
- INNER JOIN: matching rows
- LEFT JOIN: all from left + matches from right
- RIGHT JOIN: all from right + matches from left
- FULL OUTER JOIN: all from both
18. What are aggregate functions
- Perform calculations on groups
- COUNT, SUM, AVG, MIN, MAX
- Example: SELECT AVG(salary) FROM employees
19. What is a subquery
- Query inside another query
- Used in SELECT, WHERE, FROM
- Example: SELECT * FROM users WHERE age > (SELECT AVG(age) FROM users)
20. Interview tip you must remember
- Explain query execution order: FROM → WHERE → GROUP BY → HAVING → SELECT → ORDER BY
- Use simple examples with 2-3 rows
- Practice on sample datasets like employees/orders
0 Comments