Code Lab BetaBUILD. RUN. DISCOVER.
Code Lab is in development. New lessons and improvements are on the way. Progress can be deleted.
Loading…
Code Lab is in development. New lessons and improvements are on the way. Progress can be deleted.
Loading…
Code Lab is in development. New lessons and improvements are on the way. Progress can be deleted.
SQL / POSTGRESQL / Learn / LESSON 08
After LEFT JOIN, o.id IS NULL finds customers without orders. Use id because a real order cannot have a NULL primary key. total = 0 is different from an absent order.
Small example:
SELECT c.id FROM customers c LEFT JOIN orders o ON c.id = o.customer_id WHERE o.id IS NULL;
Working tables for this task. Grading also uses other datasets with the same schema.
| id | name | category | price | stock |
|---|---|---|---|---|
| 1 | Book Light | books | 12 | 8 |
| 2 | Bowl | home | 20 | 4 |
| 3 | Lamp | home | 30 | 2 |
| 4 | Notebook | books | 10 | 12 |
| 5 | Box | NULL | 5 | 0 |
| 6 | Book Ocean | books | 20 | 1 |
| 7 | Mug | home | 30 | 3 |
| 8 | Pen | books | 19.99 | 7 |
| 9 | Cable | tools | 20.01 | 5 |
| 10 | Clip | tools | 9.99 | 3 |
| 11 | Fan | home | 30.01 | 1 |
| 12 | Book Edge | books | 20.01 | 2 |
| id | name | city |
|---|---|---|
| 1 | Mira | Sofia |
| 2 | Niko | Varna |
| 3 | Iva | Sofia |
| 4 | Alex | NULL |
| 5 | Boris | Plovdiv |
| 6 | Lena | Burgas |
| id | customer_id | total |
|---|---|---|
| 1 | 1 | 20 |
| 2 | 1 | 30 |
| 3 | 2 | 10 |
| 4 | 3 | 0 |
| 5 | 6 | 40 |
Expected columns: name · row order is not required
Expected result (displayed data):
Loading…After LEFT JOIN, o.id IS NULL finds customers without orders. Use id because a real order cannot have a NULL primary key. total = 0 is different from an absent order.
QUERY STANDBY
›