Question

I am using mysql and have 2 tables

table1

id
324
325
328
350
420
650
850
950

table2

id    mapping_id
324   1
325   2
328   3
350   4
420   5
650   1
850   2

I want to produce a list of all the DISTINCT field mapping_ids that are missing for the ids in table one. For example id 850 has a mapping_id of 2 so is missing 1,3,4,5 and id 950 is not even in table 2 and so is missing 1,2,3,4,5. This should give me a distinct list of 1,2,3,4,5.

I have tried various LEFT JOIN queries but cannot get the results I need. Thanks in advance.

Answer 1

You could build a matrix of id - mapping combinations using a cross join. A not in subquery can determine which parts of the matrix are empty:

select  *
from    table1 t1
cross join
        (
        select  distinct mapping_id
        from    table2
        ) mappings
where   not exists
        (
        select  *
        from    table2 t2
        where   t2.id = t1.id
                and t2.mapping_id = mappings.mapping_id
        )