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HAVING

In MySQL, the HAVING clause is used in conjunction with the GROUP BY clause to filter the results of a query based on aggregate functions.

The HAVING clause is applied after the GROUP BY clause and allows you to specify conditions for groups of rows. It is primarily used with aggregate functions like SUM, COUNT, AVG, etc.

Syntax

SELECT column_name
FROM table_name
WHERE condition
GROUP BY column_name
HAVING condition

Example

Suppose we have a Students table as shown below:

Now, let's say you want to find courses where the number of students who scored above 79 is greater than 1. You can use the HAVING clause for this:

SELECT course, COUNT(student_id) as total_students
FROM Students
WHERE score > 79
GROUP BY course
HAVING total_students > 1;

java

SELECT course, COUNT(student_id) as total_students
FROM Students
WHERE marks > 79
GROUP BY course
HAVING total_students > 1;

In this example:

We select the course column and count the number of students who scored above 79 using COUNT(student_id) for each course.
We use WHERE score > 79 to filter out individual rows where the score is not greater than 79 before grouping.
We use GROUP BY course to group the data by the course.
The HAVING clause filters out groups where the number of students is not greater than 1.

The result of this query would be:

This result includes both "Math" and "English" courses because they have more than one student who scored above 79.

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📝 My notes

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