Unique Values with DISTINCT

Which genres does the shop have? SELECT genre FROM games; gives 20 rows, one for each game, and most genres are there several times. DISTINCT, right after SELECT, keeps each different row only once:

SELECT DISTINCT genre
FROM games
ORDER BY genre;

Eight genres instead of twenty rows. DISTINCT itself doesn't sort anything: the genres are in alphabetical order thanks to ORDER BY, and without it, they could come in any order. (RPG comes before Racing, because capital letters come before lowercase ones, as you saw at the beginning of this lesson.)

With several columns, DISTINCT removes only the rows that are the same in all of them. Which prices did each game sell for?

SELECT DISTINCT game_id, price
FROM order_items
ORDER BY game_id, price;

Most games are there once, but some twice, such as game 5, Shadow Ninja: it sold for $25, and for $15 on Black Friday. What counts is the combination of the values in the row, not each column on its own. NULL counts as a value too: if several rows have NULL, DISTINCT keeps one of them.

DISTINCT is written once, right after SELECT, and it applies to the whole row. Typical mistake: putting it in front of one of the columns. SELECT game_id, DISTINCT price FROM order_items; ends with the error near "DISTINCT": syntax error.