SQL Quiz: 20 Questions for Interview Prep (2026)

Q1. What does SQL stand for?

✅ Answer: Structured Query Language

💡 SQL is the standard language for storing, retrieving, and managing data in a relational database.

Q2. Which statement is used to retrieve data from a table?

✅ Answer: SELECT

💡 SELECT reads data. For example, `SELECT name FROM users;` returns the name column from the users table.

Q3. Which clause filters rows based on a condition?

✅ Answer: WHERE

💡 WHERE keeps only rows that meet a condition, e.g. `WHERE age > 18`. It filters rows before any grouping.

Q4. How do you sort query results?

✅ Answer: ORDER BY

💡 ORDER BY sorts results ascending by default; add DESC for descending, e.g. `ORDER BY created_at DESC`.

Q5. Which keyword removes duplicate rows from results?

✅ Answer: DISTINCT

💡 `SELECT DISTINCT country FROM users;` returns each country only once, removing duplicates.

Q6. Which statement adds a new row to a table?

✅ Answer: INSERT INTO

💡 `INSERT INTO users (name) VALUES ('Sara');` adds a new record to the users table.

Q7. Which statement modifies existing rows?

✅ Answer: UPDATE

💡 UPDATE changes data in existing rows. Always pair it with a WHERE clause, or every row will be updated.

Q8. Which statement removes rows from a table?

✅ Answer: DELETE

💡 `DELETE FROM users WHERE id = 5;` removes matching rows. Without WHERE, it deletes all rows.

Q9. What does the LIMIT clause do?

✅ Answer: Restricts the number of rows returned

💡 `SELECT * FROM users LIMIT 10;` returns at most 10 rows. SQL Server uses TOP instead of LIMIT.

Q10. Which operator checks whether a value is within a range?

✅ Answer: BETWEEN

💡 `WHERE age BETWEEN 18 AND 30` matches values from 18 to 30 inclusive.

Q1. What is a PRIMARY KEY?

✅ Answer: A column (or set of columns) that uniquely identifies each row

💡 A primary key must be unique and cannot be NULL. Each table can have only one primary key.

Q2. What is a FOREIGN KEY?

✅ Answer: A column that references the primary key of another table

💡 Foreign keys enforce referential integrity, linking related tables (e.g. an orders table referencing a customers table).

Q3. What does an INNER JOIN return?

✅ Answer: Only rows that have matching values in both tables

💡 An INNER JOIN combines rows from two tables where the join condition is met, dropping unmatched rows.

Q4. What is the difference between INNER JOIN and LEFT JOIN?

✅ Answer: LEFT JOIN returns all rows from the left table plus matches from the right (NULLs where none match)

💡 INNER JOIN keeps only matched rows; LEFT (OUTER) JOIN keeps every left-table row even when there is no match on the right.

Q5. What does the GROUP BY clause do?

✅ Answer: Groups rows that share a value so aggregate functions can be applied per group

💡 `SELECT country, COUNT(*) FROM users GROUP BY country;` counts users per country.

Q6. What is the difference between WHERE and HAVING?

✅ Answer: WHERE filters rows before grouping; HAVING filters groups after aggregation

💡 You cannot use an aggregate like COUNT() in WHERE, but you can in HAVING, e.g. `HAVING COUNT(*) > 5`.

Q7. Name the common SQL aggregate functions.

✅ Answer: COUNT, SUM, AVG, MIN, and MAX

💡 These summarise data across rows, e.g. `AVG(salary)` returns the average salary in a group.

Q8. What is an index and why is it used?

✅ Answer: A data structure that speeds up row lookups

💡 Indexes make reads faster but slightly slow down writes and use extra storage. They are ideal on columns used in WHERE and JOIN conditions.

Q9. What is database normalization?

✅ Answer: Organising tables to reduce data redundancy and improve integrity

💡 Normalization splits data into related tables (e.g. 1NF, 2NF, 3NF) so the same information is not stored in multiple places.

Q10. What is the difference between DELETE, TRUNCATE, and DROP?

✅ Answer: DELETE removes selected rows; TRUNCATE removes all rows quickly; DROP removes the entire table

💡 DELETE can use WHERE and is logged row by row. TRUNCATE clears all rows fast with no WHERE. DROP deletes the table structure itself.

Are these SQL questions good for interviews?

Yes. Queries, JOINs, keys, GROUP BY vs HAVING, indexes, and normalization are among the most commonly asked SQL topics in data-analyst, backend, and data-engineering interviews.

Do these questions apply to MySQL and PostgreSQL?

Almost entirely. They cover standard SQL, which works across MySQL, PostgreSQL, SQL Server, and SQLite. Only small syntax details differ, such as LIMIT (MySQL/PostgreSQL) versus TOP (SQL Server).

What is the best way to practise SQL?

Create a small sample database and write real queries, or use a free online SQL sandbox. Practising by actually running queries cements the concepts far better than reading alone.