GROUP BY

GROUP BY splits rows into groups and runs the aggregate once per group. Compare: how many customers live in each city?

Examplesql

Average price per category, sorted:

Examplesql

Every column in SELECT must either be in GROUP BY or be inside an aggregate function.

Quick check

What does `SELECT city, COUNT(*) FROM customers GROUP BY city` return?

One row with the total countOne row per city with how many customers it hasEvery customer once

Your turn: write the code

Return each customer_id from orders with how many orders they placed, sorted by customer_id.

Finish the quick check (and any code exercise) to complete this lesson, or mark it done yourself.

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