The school is very grateful for your work, and now there is a new task for us.

It turns out that some students took additional exams when they were supposed to take only one. The school suspects them of cheating because each student should have only one grade.

We've been tasked with getting the last names of these students and passing them on to the school administration for them to take whatever action they deem necessary.

Let's think together about how we can do this. You could start by considering that we can do this using a WHERE clause, and it would look something like this:

But, as you can see, we get an error indicating that we cannot use aggregate functions inside a WHERE clause. This is where we'll need the HAVING clause.

Let's understand what it is and how to use it, using an example from our employee table.

Let's say we need to retrieve the departments where employees' average salary is below $70,000 per year.

To achieve this, we'll need to use an aggregate function and the HAVING clause.

Let's see how we can do this:

We received one department in response using the HAVING clause, where we set a condition for the column by which we grouped the data.

Note

To use data aggregation within the HAVING clause, we need to have data grouping in our query. As in the query above, we grouped the data by the department column.

Let's look at the more generalized syntax of the HAVING clause and when it's best to use it:

Let's also briefly understand the main difference between WHERE and HAVING clauses and when to use each of them:

1. The WHERE clause is used before data aggregation, while the HAVING clause is used after data aggregation;
2. The WHERE clause is written before GROUP BY, while the HAVING clause is written after GROUP BY.

These are the two main differences you need to remember for successful use of the HAVING clause. Now, let's return to the task given to us by the school.

Here is the preview of a student_grades table we are working with: