Question

I have these three (simplified) tables for a contest application:

**user**
user_id    

**contest**
contest_id

**contest_entry**
contest_id
user_id

There are several contests, and each user can enter each contest once (creating a contest_entry record).

Thus far there have been 5 contests, so I’d like to get a count of how many users have entered 0, 1, 2, 3, 4, or 5 contests. An example of the format I’m looking for is this:

num_contests_entered      num_users
0                         102
1                         87
2                         345
3                         254
4                         567
5                         489

I am completely stumped on the SQL needed to achieve this, so any help is greatly appreciated!

Best,
Chris

EDIT: To clarify, an example of what I am looking for is

102 users entered 0 contests
87 users entered 1 contest
345 users entered 2 contests 

etc..

Answer 1

You can write:

SELECT num_contests_entered,
       COUNT(1) num_users
  FROM ( SELECT COUNT(ce.contest_id) num_contests_entered
           FROM user u
           LEFT
          OUTER
           JOIN contest_entry ce
             ON ce.user_id = u.user_id
          GROUP
             BY u.user_id
       ) t
 GROUP
    BY num_contests_entered
 ORDER
    BY num_contests_entered
;

The subquery finds, for each user, how many contests they entered; the outer query groups users by that count, thereby determining the number of users that entered that many contests.

(Disclaimer: not tested.)