SQL JOIN多对多

Question

Sorry about the minimalistic title but I don't know how to describe it in short. I have three tables:

The table of groups

ID | Genre
-----------------
1  | Action
2  | Adventure
3  | Drama

Many to many table

GroupID | ElementID
-----------------
    3   |    1
    1   |    2
    2   |    2
    2   |    3
    3   |    3

And the table of elements

ID | Element
-----------------
1  | Pride and Prejudice
2  | Alice in Wonderland
3  | Curious Incident Of A Dog In The Night Time

All is fine and very simple. The SELECT I am trying to achieve is the following

ID | Element                                         |  Genre
-------------------------------------------------------------
1  | Pride and Prejudice                             | Drama
2  | Alice in Wonderland                             | NULL
3  | Curious Incident Of A Dog In The Night Time     | Drama

I want to select all the elements from the table Elements and set the genre field to Drama or null .

I'm trying to do this in MySQL .

Thank you in advance

Answer 1

It's possible with this little trick (OUTER JOIN on the many-to-many table, with the constraint that the GroupID has to be 3 (for Drama)

http://sqlfiddle.com/#!2/b7c18/2

SELECT elements.ID, elements.Element, groups.Genre
  FROM elements
LEFT OUTER JOIN group_elements
  ON elements.ID = group_elements.ElementID
 AND group_elements.GroupID = 3
LEFT OUTER JOIN groups
  ON group_elements.GroupID = groups.ID

LEFT OUTER JOIN means : take all the lines from the tables that preceded (the ones that are on the LEFT hand side of the LEFT OUTER JOIN , if you will), even if there's no lines corresponding to them in the following tables. The condition ON elements.ID = group_elements.ElementID AND group_elements.GroupID = 3 says that if we find anything that matches our ElementID, it also must be a drama (GroupID = 3). We then do another LEFT OUTER JOIN on the groups table, which enables us to display the Genre column, or NULL if the element was not a drama.