Lesson 22: Using the HAVING clause to filter aggregate functions.
By Alan Hylands — 2 minute read
Houston, we have a problem.
SQL queries run in a standard order. First the FROM and JOINs (check out Lesson 26 for Joins) are executed so SQL knows what set of data to do the rest of the work on. Next is the WHERE clause so it can filter out individual rows that don't meet the criteria.
After this SQL goes to the GROUP BY clause to group up the data based on common values specified. Which leaves us with a conundrum if we want to filter on the aggregate functions the** GROUP BY** is helping us with. WHERE is no good to us as it's already shot it's bolt in the running order.
So the HAVING clause was introduced to SQL to allow us to apply filters and constraints to the grouped data and aggregate functions.
Where does it come in our SQL query?
We bring in the HAVING clause AFTER the GROUP BY clause.
The HAVING clause in this query can bring in the aggregate function COUNT and filters out any Account Types that have less than 100 Account IDs associated with them in the Accounts table.
We only want to count the number of superheroes by nationality when there are more than one of that nationality present. Make sense? No solos.
You'll use COUNT, GROUP BY and HAVING to make this work.