Mysql count group by having

sql

subqueries

sql-aggregation

group-by

byAnton Shumikhin·Dec 21, 2024

Searching for a quick way to filter counts in MySQL? Use GROUP BY and HAVING. Here's a quick snippet:

SELECT your_column, COUNT(*) AS total
FROM your_table
GROUP BY your_column
HAVING total > your_threshold;

What are these placeholders? Replace your_column with your field to group, your_table with your source table, and your_threshold with the minimum count you need. This code serves you those your_column values which have a count that goes beyond your_threshold.

SQL Aggregation: GROUP BY with HAVING

Overview

GROUP BY was baked into SQL to provide us the capability to aggregate data, and COUNT(*) compliments it by being the proverbial nail. What’s in name if not for the count, right? However, remember not all groups deserve the gala. Only the ones that meet our criteria are invited to the party and HAVING is the bouncer who checks the guest list.

Example Implementation: Movie Genres

We are going to sink our teeth into a real-world problem; a database of movies. Each movie can have multiple genres and we want to determine how many movies bear precisely four genres. Prepared to be awed by subqueries and HAVING working in perfect harmony:

SELECT COUNT(*) AS TotalMoviesWithFourGenres
FROM (
    SELECT movie_id
    FROM movie_genres
    GROUP BY movie_id
    HAVING COUNT(genre) = 4  -- It's 4 for sure, no less, no more!
) AS SubQuery;

Our inner query neatly groups movies by their IDs and, like a hawk, filters the groups having precisely four genres. The outer query is the MVP who does the final count of movies fitting our criteria. Bravo, team!

How to win a battle with complex scenarios

When faced with a formidable opponent like multiple GROUP BY, JOINs, varying aggregates; the wise warrior uses GROUP BY and HAVING with caution in its arsenal.

Focus: Count movies, not cat videos

Don't lose focus if genres start to feel tempting! A common pitfall is to get confused and start counting genres instead of movies. Stick to our goal, which is counting movies. There is no glory in losing focus.

The Rule of “One movie, one group”

To ensure victory in the war against erroneous data representation, each movie ID needs to be grouped individually before preparing the final tally. And although subqueries tag along additional computation, they are the tactical choice when it comes to ensuring precise grouping.

Show your finesse by using Advanced Techniques

Be the speed: Indexing

Ever watched a Fast and Furious movie? Indexing does to your column what the nitro boost does to those cars. Ensure that indexes are used for columns that are involved in JOINs or WHERE conditions. It's not cheating, it's winning.

Window Functions: The Secret Weapon

Window functions are like the secret weapon in your game console. They perform computations across rows without the need for subqueries. Consider this option when you feel like flexing your SQL biceps.

Common Pitfalls: The What-Not-To-Do Guide

The COUNT() enigma

Understanding the difference between COUNT(*), COUNT(1), and COUNT(column_name) can be the game changer. Despite their similarities, COUNT(column_name) won’t count NULL values towards total, and you'd better have this marked in your playbook.

The Cardinal Sin: Non-unique identifiers

If non-unique identifiers find their way into your query, prepare to welcome incorrect groupings and erroneous counts to your results. Beware, the cost of overlooking group cardinality is steep.

The Subquery Prequel

While subqueries are like the superhero team of SQL land, they can easily turn into supervillains if not written with caution, leaving you with sub-optimal performance. Be the hero SQL-land needs and ensure your subqueries are optimised and necessary.

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