Question

I have 4 tables, and the 4 are foreign keys in one junction table.

The problem is that I need to retrieve the jnction values that are missing.

For example:

Table A       Table B      Table C     Table D             
--------     ---------    ---------   ----------
  1              4            7          A
  2              5            8          B



 JUNCTION TABLE
-----------------
  1   4   7   A
  1   4   7   B
  1   4   8   A
  1   4   8   B
  1   5   7   A
  1   5   7   B
  1   5   8   A
  1   5   8   B      
  2   4   7   A
  2   4   7   B
  2   4   8   A
  2   4   8   B
  2   5   7   A
  2   5   7   B
  2   5   8   A
  2   5   8   B      

So if the column 1 5 8 B is missing, when I run the query it shows 1 5 8 B..

Any Ideas?

Answer 1

I would use a cross join on your source tables, then join the result of that to your junction table.

EDIT: Kudos to andy holaday for pointing out that MySQL does not have FULL OUTER JOIN.

select x.*
from (select *
      from a,b,c,d) as x
left outer join j
    on (x.a = j.a and
        x.b = j.b and
        x.c = j.c and
        x.d = j.d)
where j.a is NULL and
      j.b is NULL and
      j.c is NULL and
      j.d is NULL