More joins = slower queries. Index all join columns. Filter early with WHERE. Consider denormalization for frequently joined tables.

Index ALL join columns

Filter with WHERE to reduce rows

Join smaller tables first when possible

Use covering indexes

Check execution plans

Consider denormalization for frequent joins

Forgetting ON clause for a join

Mixing up which table needs LEFT JOIN

Not indexing all join columns

Selecting unnecessary columns

Not filtering early enough

Creating accidental cartesian products