Question

I am trying to pull out the information about each user group, while counting the number of users in each.
The counting works, however if a group does not contain any users, it does not retrieve the information – it simply gets ignored.

This is my query:

 SELECT
       g.*,
       count(u.username) as total,
       u.usergroup
  FROM
      usergroups as g,
      users as u
  WHERE
      u.usergroup = g.g_id
  GROUP BY
      g.group_name

This is a part of my “users” and “user groups” tables:

 Sample usergroups table structure
  +--------------------------+---------------+
  | Field                    | Type          |
  +--------------------------+---------------+
  | g_id                     | int(10)       |<- the user group's ID.
  | group_name               | varchar(255)  |<- the name of the user group
  +--------------------------+---------------+

  Sample users table structure
  +--------------------------+---------------+
  | Field                    | Type          |
  +--------------------------+---------------+
  | id                       | int(10)       |<- the user ID
  | username                 | varchar(255)  |<- the username
  | usergroup                | int(10)       |<- ID of the user group the user is in
  +--------------------------+---------------+

I have no idea what I have missed – but then again, I am very new to more complex queries (if you can call it that).

Any help would be very appreciated!

Answer 1

You want a left join, and you’re doing an inner join:

 SELECT
       g.*,
       count(u.username) as total,
       u.usergroup
  FROM
      usergroups as g
      left join users as u on
           g.u_id = u.usergroup
  GROUP BY
      g.u_id,
      g.group_name

Now that I have your attention, I’d like to take a minute to discuss what join conditions are. See, when you’re first learning MySQL, they teach you the syntax you use. And hey, it works great, for certain sets of problems. Unfortunately, it causes you to equate “join condition” with “where predicate,” and that’s a problem.

In this case, we’re saying, “Hey, MySQL, grab me all the stuff out of the usergroups table, and try to find a match to the users table where the u_id column from usergroups equals the usergroup column from users. If you can’t find anything, that’s okay, bring the usergroup back anyway, and just null out the columns that users brings to the table.”

Previously, you were saying, “Hey, MySQL, grab me all the stuff out of the usergroups and the users table, but only where the u_id column from usergroups matches the usergroup column from users.”

If you had done it with inner join syntax, like so:

 SELECT
       g.*,
       count(u.username) as total,
       u.usergroup
  FROM
      usergroups as g
      inner join users as u on
           g.u_id = u.usergroup
  GROUP BY
      g.u_id,
      g.group_name

You would have been saying, “Hey, MySQL, grab me everything from the usergroups table, and then go find everything in the users table where u_id equals usergroup. If you don’t find a match, I don’t want to see that usergroup show up in the results.

So, to be clear, the where predicates filter the result set, whereas the join conditions map one table to another.