Find rows without corresponding second row in MySQL

Question

Given the following table:

type | area | shelf
-----|------|------
   1 |    a |     5
   2 |    a |     5
   2 |    a |     6
   1 |    a |     7
   2 |    a |     7
   1 |    b |     3

An area / shelf combination entry with type 2 always needs a corresponding entry with type 1. Type 1 can exist on its own (eg last row).

How can I find orphan type 2 rows (rows with type 2 without corresponding row with type 1) such as the third row?

Answer 1

Are you looking for this..

   SELECT t2.*
   FROM yourtable t1 
            RIGHT JOIN yourtable t2 ON t1.area = t2.area 
                                    AND t1.shelf=t2.shelf 
                                    AND t1.`type`=1 
                                    AND t2.`type`=2
   WHERE t1.`type` IS NULL

Answer 2

You can do this with aggregation and a having clause:

select shelf, area
from t
group by shelf, area
having sum(type = 2) > 0 and -- at least one type 2
       sum(type = 1) = 0;     -- no type 1