Course Content
MYSQL Tutorial
About Lesson

HAVING Clause

In the realm of MySQL queries, the HAVING clause plays a crucial role, allowing for more refined data retrieval. Unlike the WHERE clause that filters rows before grouping, HAVING filters aggregated results. This post delves into the nuances of the HAVING clause and its practical applications in MySQL.

What is the HAVING Clause?

The HAVING clause is utilized in SQL queries to filter the results of aggregate functions applied to grouped data. While the WHERE clause filters rows before aggregation, HAVING filters the results after grouping has taken place. This makes it especially useful when working with GROUP BY statements.

Syntax

The basic syntax of the HAVING clause is as follows

SELECT column1, aggregate_function(column2)
FROM table
GROUP BY column1
HAVING aggregate_function(column2) condition;

Practical Examples

Example 1: Filtering by Count
SELECT department, COUNT(employee_id) as employee_count
FROM employees
GROUP BY department
HAVING employee_count > 5;

In this example, the HAVING clause filters out departments with fewer than six employees.

Example 2: Filtering by Average
SELECT department, AVG(salary) as avg_salary
FROM employees
GROUP BY department
HAVING avg_salary > 50000;

Here, the HAVING clause is used to retrieve departments with an average salary exceeding $50,000.

Tips for Using the HAVING Clause Effectively

  1. Understand Aggregate Functions: Since the HAVING clause operates on the results of aggregate functions, a solid understanding of functions like COUNT, AVG, SUM, etc., is essential.

  2. Combine with GROUP BY: The HAVING clause is most powerful when used in conjunction with the GROUP BY statement, allowing for granular control over aggregated data.

  3. Careful with Performance: While the HAVING clause is powerful, it should be used judiciously as it can impact query performance, especially on large datasets.