How to create fast database queries

Archive for August 25th, 2009

PostgreSQL: selecting items that belong to all categories

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From Stack Overflow:

I've got two tables in SQL, one with a project and one with categories that projects belong to, i.e. the JOIN would look roughly like:

Project Category
Foo Apple
Foo Banana
Foo Carrot
Bar Apple
Bar Carrot
Qux Apple
Qux Banana

What I want to do is allow filtering such that users can select any number of categories and results will be filtered to items that are members of all the selected categories.

For example, if a user selects categories Apple and Banana, projects Foo and Qux show up.

If a user select categories Apple, Banana, and Carrot then only the Foo project shows up.

A usual approach to this problem involves GROUP BY and COUNT. However, this can be made more efficient.

Let's create a sample table:

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Written by Quassnoi

August 25th, 2009 at 11:00 pm

Posted in PostgreSQL