MySQL rejoint et COUNT (*) d'une autre table

Question

J'ai deux tables: groups et group_members

.

Le tableau de groups contient toutes les informations pour chaque groupe, comme son ID, titre, description, etc.

Dans le tableau group_members, il énumère tous les membres qui sont en dehors de chaque groupe comme celui-ci:

group_id | user_id
1 | 100
2 | 23
2 | 100
9 | 601

En fait, je veux à la liste des groupes TROIS sur une page, et je ne veux que la liste des groupes qui ont plus de quatre membres. A l'intérieur de la boucle de <?php while ?>, je veux alors quatre membres qui sont en dehors de ce groupe. Je vais avoir aucun mal à la liste des groupes, et la liste des membres dans une autre boucle interne, je ne peux pas préciser les groupes afin que seuls ceux qui ont plus de 4 membres montrent.

Quelqu'un sait comment faire cela? Je suis sûr que ce soit avec MySQL rejoint.

Answer 1

MySQL use HAVING statement for this tasks.

Your query would look like this:

SELECT g.group_id, COUNT(m.member_id) AS members
FROM groups AS g
LEFT JOIN group_members AS m USING(group_id)
GROUP BY g.group_id
HAVING members > 4

example when references have different names

SELECT g.id, COUNT(m.member_id) AS members
FROM groups AS g
LEFT JOIN group_members AS m ON g.id = m.group_id
GROUP BY g.id
HAVING members > 4

Also, make sure that you set indexes inside your database schema for keys you are using in JOINS as it can affect your site performance.

Answer 2

SELECT DISTINCT groups.id, 
       (SELECT COUNT(*) FROM group_members
        WHERE member_id = groups.id) AS memberCount
FROM groups

Answer 3

Your groups_main table has a key column named id. I believe you can only use the USING syntax for the join if the groups_fans table has a key column with the same name, which it probably does not. So instead, try this:

LEFT JOIN groups_fans AS m ON m.group_id = g.id

Or replace group_id with whatever the appropriate column name is in the groups_fans table.

Answer 4

Maybe I am off the mark here and not understanding the OP but why are you joining tables?

If you have a table with members and this table has a column named "group_id", you can just run a query on the members table to get a count of the members grouped by the group_id.

SELECT group_id, COUNT(*) as membercount 
FROM members 
GROUP BY group_id 
HAVING membercount > 4

This should have the least overhead simply because you are avoiding a join but should still give you what you wanted.

If you want the group details and description etc, then add a join from the members table back to the groups table to retrieve the name would give you the quickest result.