Question

How would I query the following?

User.name and Groups.description, 
from the User table and Groups table, 
where User.id_user = User_Group.id_user, and Groups.id_groups = User_Group.id_groups 

I read about UNION and Joins, but UNION requires the tables to have the same number of columns, while the JOINs seem to work only with 2 tables?? Is it possible to combine UNION and Joins?

SELECT User.Name
FROM User
FULL JOIN User_Groups
ON User.Id_user= User_Groups.Id_user
UNION
SELECT Group.Description
FROM Group
FULL JOIN User_Groups
ON Group.Id_group= Groups.Id_group

Would the above code be right??

Answer 1

Try this simplified query:

SELECT u.Name
FROM   User u
JOIN   User_Groups USING (Id_user)

UNION  ALL
SELECT g.Description
FROM   Group g
JOIN   User_Groups USING (Id_group)

• Use UNION ALL instead of UNION, because UNION would eliminate any rows with the same values.

• The result of a UNION has only one header, the column names are taken from the fist SELECT in the query. So the column name will be “Name” in your case.

• USING is just a syntactical simplification for ON when the joining columns share the same name(s) and occur only once left and right of the join.

• Table aliases User AS u or simply User u simplify later references in the code.

• You can JOIN as many tables as you like, this is not limited to two tables. I suspect, what you really want is this:

SELECT u.Name, g.Description
FROM   User u
JOIN   User_Groups USING (Id_user)
JOIN   Group g USING (Id_group)

• But there is no FULL [OUTER] JOIN in MySQL. Other RDBMS have that, like PostgreSQL. Read here.
  I suspect, that’s not what you wanted to begin with and replaced it with a plain [INNER] JOIN. Read more in the manual here.