GROUP BY comblines the output into groups of rows that matches on
one or multiple values. Also, GROUP BY usually used with aggregate functions
like COUNT, MIN, MAX, SUM, and AVG.
-- count number of products for each company
SELECT COUNT(id), company
FROM products
GROUP BY company;
-- +-------+---------------------------------+
-- | count | company |
-- |-------+---------------------------------|
-- | 1 | Edwards, Fox and Valentine |
-- | 2 | Hale-Bryan |
-- | 1 | Alexander Inc |
-- | 1 | Stark and Sons |
-- | 2 | Martin, Baker and Henderson |
-- ...
-- get number of orders of each user and order them
-- by the number of orders
SELECT COUNT(id), user_id
FROM orders
GROUP BY user_id
ORDER BY count desc;
-- +-------+---------+
-- | count | user_id |
-- |-------+---------|
-- | 8 | 332 |
-- | 7 | 367 |
-- | 7 | 456 |
-- | 6 | 192 |
-- | 6 | 307 |
-- | 6 | 194 |
-- ...
