Arcade

Join Room

Phase 05 · Backend EngineeringPlayable

Two tables, one exact result. Pick the join, and where the filter goes.

0 / 5 cleared

Level 1 · Only the matches

Every order, with the name of the customer who placed it. Orders belonging to nobody should not appear, and customers with no orders should not either.

users u

id	name
1	Ada
2	Grace
3	Alan

orders o

id	user_id	total	status
11	1	40	paid
12	1	25	refunded
13	2	90	paid
14	9	10	paid

Your query

SELECT *
FROM users u
INNER JOIN orders o
  ON u.id = o.user_id

Your result3 rows · matches

u.id	u.name	o.id	o.user_id	o.total	o.status
1	Ada	11	1	40	paid
1	Ada	12	1	25	refunded
2	Grace	13	2	90	paid

Target3 rows

u.id	u.name	o.id	o.user_id	o.total	o.status
1	Ada	11	1	40	paid
1	Ada	12	1	25	refunded
2	Grace	13	2	90	paid

INNER keeps only rows that matched. Notice what it silently removed: Alan, who has never ordered, and order 14, whose user_id points at a customer that does not exist. Both are the kind of thing you want to know about.

The result table is computed from the rows above it by the same rules a database applies, so the query printed here would produce exactly these rows against exactly these tables.