Joins

Normalised databases split data across tables. A JOIN stitches them back together by matching a key — usually a foreign key pointing at another table's primary key.

An INNER JOIN (the default) keeps only rows that match on both sides. A LEFT JOIN keeps every row of the left table and fills the right-hand columns with NULL where nothing matched — that is how you find customers with no orders.

Joins and aggregation combine naturally: join first to widen the rows, then GROUP BY to summarise them.