- What is the difference between WHERE and HAVING in SQL?
Answer:
The main difference is WHERE filters rows before grouping, while HAVING filters groups after aggregation.
WHERE
- Used with SELECT, UPDATE, DELETE
- Filters individual rows
- Cannot use aggregate functions like SUM() or COUNT()
Example
SELECT *
FROM employees
WHERE salary > 50000;
HAVING
- Used with GROUP BY
- Filters grouped data
- Works with aggregate functions
Example
SELECT department, COUNT()
FROM employees
GROUP BY department
HAVING COUNT() > 5;
- What is the difference between INNER JOIN and LEFT JOIN?
Answer:
INNER JOIN
- Returns only matching records from both tables.
LEFT JOIN
- Returns all records from the left table and matching records from the right table.
- If no match is found, NULL values are returned.
Example
SELECT e.name, d.department_name
FROM employees e
INNER JOIN departments d
ON e.department_id = d.id;
- What is a Primary Key in SQL?
Answer:
A Primary Key is a column (or group of columns) that uniquely identifies each row in a table.
Key features:
- Must contain unique values
- Cannot contain NULL values
- Only one primary key per table
Example
CREATE TABLE users (
id INT PRIMARY KEY,
name VARCHAR(50),
email VARCHAR(100)
);
- What is the difference between DELETE, TRUNCATE, and DROP?
Answer:
DELETE
- Removes specific rows
- Can use WHERE
- Can be rolled back
DELETE FROM employees WHERE id = 10;
TRUNCATE
- Removes all rows
- Faster than DELETE
- Cannot use WHERE
TRUNCATE TABLE employees;
DROP
- Deletes the entire table structure and data
DROP TABLE employees;
- What is a Subquery in SQL?
Answer:
A Subquery is a query inside another query. It is used to retrieve data that will be used in the main query.
Example
SELECT name
FROM employees
WHERE salary > (
SELECT AVG(salary)
FROM employees
);
This query finds employees whose salary is greater than the average salary.
✅ Want to master SQL step by step?
Check out the complete SQL tutorial here:
https://www.quipoin.com/tutorial/sql
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