I need to check for a value in either table_one or table_two.

In table_one the value is stored as a foreign_key id referencing table_three and in table_two it is stored directly.

Pseudo code:

SELECT T1.*
    FROM table_one T1
    JOIN table_three T3 ON T3.id = T1.foreign_key
    WHERE T3.field = 'value'
OR
SELECT T2.*
    FROM table_two T2
    WHERE T2.field = 'value';

I also need to know from WHICH table the value is actually selected.

Unfortunately the unnatural syntax of SQL makes this practically impossible to ‘figure out’ by one self.