Learn HAVING Clause

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:


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SELECT student_surname
FROM student_grades
WHERE COUNT(grade) > 1

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 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:


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SELECT department
FROM employees
GROUP BY department
HAVING AVG(salary) < 70000

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:

SELECT column1, column2 --(optional)
FROM table
GROUP BY column1
HAVING AGG(column_n) --(condition)

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

The WHERE clause is used before data aggregation, while the HAVING clause is used after data aggregation;
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.

Task

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Your task will be to fetch the last names of students who have multiple grades for the school.

You only need to retrieve the last names of the students; there's no need to include the count of their grades in the response. Use the HAVING clause and the COUNT() aggregate function to accomplish this task. Then, sort the surnames in the alphabetical order.

Note:

You should have only one column with last names in the result.

Brief Instructions

Retrieve the student_surname column.
Group the data by student_surname.
Use the HAVING clause to filter the results based on COUNT(grade) > 1.
Sort the results by student_surname.

Solution

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Section 1. Chapter 6

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