Question

I have 3 tables, User, Groups , and UserGroups.

UserGroups hold the User id_user and Group id_group.

User has id_user, and Group has id_group.

My question would be:

How do I pull data from all 3 tables to achieve the following:

1. Listing All Groups in Group Table
2. Listing Groups in the UserGroup Table that has been selected
3. Listing Users that ARE NOT in UserGroups in the User Table.
4. Listing Users that belong to a Group in the UserGroup Table.

I have the following SQL statements, but I would wish to combine them into 1 sql statement. Is it possible??

select * from grupos

and

SELECT DISTINCT usuarios.id_usuario
FROM usuarios
WHERE usuarios.id_usuario
NOT IN (SELECT DISTINCT id_usuario FROM usuarios_grupos)

Whereby, the first query displays:

Groups

1. John
2. Tim
3. Jane

The 2nd query shows the following:

User
 1. John
 2. Jane
 3. Tom

User Group
 Empty

After selection of John:

User
 2. Jane
 3. Tom

User Group
 1. John

Instead of having the following (which I have now):

User

 1. John
 2. Jane
 3. Tom

After Selection of John:

UserGroup

 1. John

Like what I have demonstrated above, if I select a Group and a User, and add it to User Groups. User Groups will display “Group, User” . (E.g. Development, Tom).

However, the Groups data table will still be shown as the following:

1. Development
2. Web
3. Projects

The User data table will be “updated” instead:

1. Tim
2. Jane.

I tried the following, but I didn’t get the results I wanted:

SELECT grupos.id_grupo, usuarios.id_usuario
FROM  usuarios_grupos
INNER JOIN
grupos
ON
usuarios_grupos.id_grupo = grupos.id_grupo
INNER JOIN
usuarios
ON
usuarios_grupos.id_usuario = usuarios.id_usuario
WHERE
usuarios.id_usuario
NOT IN (SELECT DISTINCT id_usuario FROM usuarios_grupos)

Answer 1

The simplest way to list the groups that have users belonging to them would be:

SELECT DISTINCT id_grupo FROM usuarios_grupos

The simplest way to list the groups that don’t have users belonging to them would be:

SELECT DISTINCT id_grupo 
FROM grupos
WHERE id_grupo
NOT IN (SELECT DISTINCT id_grupo FROM usuarios_grupos)

EDIT: To list the Users that don’t belong in Groups, and all Groups that have Users in them, you can union the results of the queries that return each of these sets –

SELECT DISTINCT id_usuario
FROM usuarios
WHERE id_usuario
NOT IN (SELECT DISTINCT id_usuario FROM usuarios_grupos)
UNION ALL
SELECT DISTINCT id_grupo FROM usuarios_grupos

Or you can do this in a single query:

SELECT DISTINCT COALESCE(ug.id_grupo, u.id_usuario)
FROM usuarios u
LEFT JOIN usuarios_grupos ug ON u.id_usuario = ug.id_usuario

(This last query assumes that there is no overlap between group and user ID sets.)