Join-aggregate queries can be expensive. Index join columns. Filter with WHERE before GROUP BY. Consider pre-aggregated tables for frequent queries.

Index all join columns

Filter with WHERE to reduce rows before GROUP BY

Use covering indexes

Consider materialized views for complex aggregations

Pre-aggregate in subqueries when appropriate

Using COUNT(*) instead of COUNT(column) with LEFT JOIN

Double-counting from incorrect joins

Not including all SELECT columns in GROUP BY

Filtering aggregates with WHERE instead of HAVING

Not handling NULL in aggregations