Question

I have a parent table A that have 2 foreign keys (from tables B and C), but it can have just one foreign key at a time. Example:

SELECT a.evi, a.time, a.date, 
       a.number, a.descr, a.x, 
       a.y, a.z,  a.FK_tableB, a.FK_tableC, 
       b.brand, b.type, 
       b.color, b.model, c.name, c.lastname, 
  FROM tableA a, 
       tableB b, 
       tableC c  
 WHERE (PK_tableA = 100 AND PK_tableB = FK_tableB)      
    OR (PK_tableA = 100 AND PK_tableC = FK_tableC)

(This is not working, obviusly)

How do I return data when just one of the where clauses is true.

Answer 1

It appears you want to do a “Exclusive OR” (XOR) on your query.

Since SQL doesn’t have XOR, you could try something like:

create table a
( a_id int, b_id int, c_id int);

create table b
( b_id int);

create table c
( c_id int);

insert into a (a_id, b_id, c_id) values (1, 1, 1);
insert into a (a_id, b_id, c_id) values (2, NULL, 2);
insert into a (a_id, b_id, c_id) values (3, 2, NULL);
insert into a (a_id, b_id, c_id) values (4, NULL, NULL);

insert into b (b_id) values (1);
insert into b (b_id) values (2);

insert into c (c_id) values (1);
insert into c (c_id) values (2);

SELECT a.a_id, a.b_id, a.c_id, b.b_id, c.c_id
  FROM a 
  LEFT JOIN b
    ON (a.b_id = b.b_id)
  LEFT JOIN c  
    ON (a.c_id = c.c_id)
 WHERE (   (b.b_id is NOT NULL AND c.c_id is NULL) 
        OR (c.c_id is NOT NULL AND b.b_id is NULL));

See this SQLFiddle to try it out.