Let’s suppose I have a table with integer columns a and b (each pair (a,b) is unique) and following data:
a | b
-------
1 | 2
1 | 3
1 | 4
1 | 5
2 | 1
2 | 3
2 | 4
3 | 1
3 | 3
Now I need to get all the rows whose b value is not contained in the set with some other a value (i.e. they appear just once in the union of rows with the ‘a’ values in question). From these I should get (1,2), (1,5) and (2,1).
I want to include only two sets (groups with the same a value; I want to use this as part of a function, where both a values are given as parameters) at a time, so I don’t want to get (3,1) from the new data.
I have some (working) solution; I post it as an answer. However, there should be something more elegant – the query should be one and exploit the symmetricity, not two unioned queries. I thought of various solutions listed in this answer, but after some hour of playing with code I didn’t find how to make anything better and working.
