Aggregates and Grouping

Part of the MySQL course

Somebody asks how the EU region did last quarter. They do not want four thousand rows, they want one number, and if you fetch all four thousand rows into application code and add them up there, you have written a slow and fragile version of something the database does natively in a few milliseconds.

Every query so far returned one output row per input row. Aggregates flip that: they collapse many rows into a single value. Add GROUP BY and you get one such value per bucket instead of one for the whole table, which is where reports come from. ...