How to count users owning a certain game using one SQL query? - sql

Question

I have a table of games, like this:
ID | game name
1 legend of zelda
2 metal gear solid
3 resident evil
And another table of users owning those games, like this:
ID | User ID | Game ID
1 510 2
2 879 2
3 213 3
I need to make a list of games with a number of users owning them. From the above, the result would be:
legend of zelda (0 users)
metal gear solid (2 users)
resident evil (1 user)
How do I do the above using only 1 SQL query?

Answer 1

SELECT g."game name" AS name, COUNT(u.ID) AS users
FROM games AS g LEFT JOIN game_users AS u
ON g.ID = u."Game ID"
GROUP BY g."game name"
I've used SQL standard double quotes around the space-containing delimited identifiers since you didn't identify which sub-species of SQL you are using.
This gives you two columns of output - the game name and a simple count. If you want the decorative '(0 users)' and '(1 user)' notations, then you are into some more serious pain unless your DBMS provides a convenient function to handle the correct inflections for different numbers of an object in your language (apparently English - but the rules vary by language).
Doing the simple-minded computerese:
SELECT TRIM(r.name) || ' (' || r.users || ' users)'
FROM (SELECT g."game name" AS name, COUNT(u.ID) AS users
FROM games AS g LEFT JOIN game_users AS u
ON g.ID = u."Game ID"
GROUP BY g."game name") AS r
Or, slightly more sophisticated (but English-only):
SELECT TRIM(r.name) || ' (' || r.users || ' user' ||
CASE r.users WHEN 1 THEN '' ELSE 's' END || ')'
FROM (SELECT g."game name" AS name, COUNT(u.ID) AS users
FROM games AS g LEFT JOIN game_users AS u
ON g.ID = u."Game ID"
GROUP BY g."game name") AS r
However, in my book, SQL is for the data; presentation is best done in the application. Therefore, I'd probably use the first query.

Answer 2

Join them, group on game id, and select the count(*) of rows, or count(distinct users). If one guy owns the game twice, and you want that to count as only one owner, distinct users.
SELECT game.name, count(*)
FROM games
JOIN ownershipInfo
ON games.id = ownershipInfo.id
GROUP BY game.name --missing from original answer, oops
That will give you the number of rows.
Since you want to include where there are zero, do "LEFT JOIN" instead of plain join. If you want the count not of rows but distinct owners:
SELECT game.name, count(distinct ownershipInfo.ownerID)
FROM games
JOIN ownershipInfo
ON games.id = ownershipInfo.id
GROUP BY game.name --missing from original answer, oops
As jonathon points out you need a GROUP BY to get the aggregate information. And here I just took the expedient of grouping by name - usually you'd group by the game's ID, and do something more like:
SELECT max(game.id), count(distinct ownershipInfo.ownerID)
FROM games
LEFT JOIN ownershipInfo
ON games.id = ownershipInfo.id
GROUP BY games.id
That way if two games have the same name, you'd get a row for both (of course it wouldn't be self explanatory but maybe you'd also have MAX(publisher) or something to clarify which one was which)

Answer 3

SELECT g."game name" AS name, COUNT(*) AS users
FROM games AS g
INNER JOIN game_users AS u
ON g.ID = u."Game ID"
GROUP
BY g."game name"
UNION
SELECT g."game name" AS name, 0 AS users
FROM games AS g
WHERE NOT EXISTS (
SELECT *
FROM game_users AS u
WHERE g.ID = u."Game ID"
);