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SQL / POSTGRESQL / Learn / LESSON 03
GROUP BY makes a group for each key value. Aggregates run per group. Each non-aggregated selected column must be grouped. NULL values form one group.
Example: SELECT city, COUNT(*) AS total FROM customers GROUP BY city yields one row for Sofia, Varna, and for the city = NULL group when such customers exist.
Small example:
SELECT city, COUNT(*) AS total FROM customers GROUP BY city;
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: category, total · row order is not required
Expected result (displayed data):
Loading…GROUP BY makes a group for each key value. Aggregates run per group. Each non-aggregated selected column must be grouped. NULL values form one group. Example: SELECT city, COUNT(*) AS total FROM customers GROUP BY city yields one row for Sofia, Varna, and for the city = NULL group when such customers exist.
QUERY STANDBY
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