How to use the HAVING Clause in PostgreSQL with Examples

👋 Hello! Today, we’re going to take an exciting journey into the world of PostgreSQL’s HAVING clause. If you’ve mastered the GROUP BY clause and are eager to take your SQL querying to the next level, the HAVING clause is your next stop.

What Is the HAVING Clause?

The HAVING clause filters the results of aggregate functions based on a condition. It’s like the WHERE clause but for groups, not individual rows.

Basic Syntax

Here’s how you can structure a query with the HAVING clause:

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

A Practical Example

Say you want to find out which genres in your tv_series table have more than 5 seasons. Your SQL query would look like this:

SELECT genre, COUNT(seasons)
FROM tv_series
GROUP BY genre
HAVING COUNT(seasons) > 5;

HAVING with Multiple Conditions

You can also use multiple conditions with HAVING by using AND or OR. For example, if you want to find genres with a total episode count greater than 5 but less than 10, you’d write:

SELECT genre, COUNT(seasons)
FROM tv_series
GROUP BY genre
HAVING COUNT(seasons) > 5 AND COUNT(seasons) < 10;

Common Errors and Solutions

Error Message:

ERROR:  aggregate functions are not allowed in HAVING

Cause:

You’ve used an aggregate function incorrectly in the HAVING clause.

Solution:

Check to ensure that aggregate functions in the HAVING clause are also present in the SELECT clause and are used appropriately.

Error Message:

ERROR:  column "column_name" must appear in the GROUP BY clause or be used in an aggregate function

Cause:

You’ve included a column in the HAVING clause that isn’t part of the GROUP BY clause or an aggregate function.

Solution:

Make sure to only refer to columns that are part of your GROUP BY clause or are used within an aggregate function in the HAVING clause.

Wrapping Up

The HAVING clause allows you to filter grouped data, granting you a finer level of control when working with aggregate functions. It’s another valuable tool in your PostgreSQL toolkit. Go ahead and experiment using the tv_series table — you’ll find that understanding how to use HAVING effectively opens new doors in your data analysis journey.

Happy querying, and till next time! 🎉