Question

I’m building an e-shop using RDBMS, and wanted to know what is the most efficient way to represent the number of items in a particular category? So for instance, my category menu would look something like this:

• Books (212)
• MP3 players (13)
• Footwear (562)

What techniques do you use to get those numbers? I’m using PostgreSQL 9.1, but I assume the mechanics should be similar throughout all DBMSs.

Answer 1

You need GROUP BY

SELECT category, count(*)
FROM items
GROUP BY category

This will count the number of items in the table for each category.