How to put a constraint on two combined fields?

Question

I'd like to put a constraint, a check or a foreign key, on two combined fields from table1 to another field in table2. Here is what I tried, but both gave me errors:

ALTER TABLE table1
    ADD CONSTRAINT foo CHECK (field1 || field2 IN (SELECT fieldx FROM table2));

ALTER TABLE table1
    ADD CONSTRAINT foo FOREIGN KEY (field1 || field2) REFERENCES table2 (fieldx);

Is this possible? If yes, how? Beside this, is it generally possible to use subselects in CHECK Constraints?

I'm using DB2/LINUX 9.5.0.

Solution:
It seems to be only possible with an extra column, that holds the combinied value. The good thing is, Tony Andrews pointed out, there is a way to create expression generated columns in DB2.

CREATE TABLE table1 (
    field1 CHARACTER(5),
    field2 CHARACTER(5),
    fieldx CHARACTER(10) GENERATED ALWAYS AS (field1) || field2),
    CONSTRAINT FK_X FOREIGN KEY (fieldx) REFERENCES table2 (fieldx)

);

Answer 1

One possibility would be to hold a computed column on table1 i.e.

fieldx = (field1 || field2)

I don't know if DB2 supports computed (aka virtual) columns as such, but if not you can create a regular column and maintain it via a trigger. The create the foreign key constraint:

ALTER TABLE table1
    ADD CONSTRAINT foo FOREIGN KEY (fieldx) REFERENCES table2 (fieldx);

Another possibility, of course, would be to modify your table design so that the keys are held consistently: if field1 and field2 are atomic values, then they should appear as such in table2, not as a concatenated value (which more or less breaks 1NF).

Answer 2

You don't, the foreign key must have the same number of columns as the parent key, also consider that keys need indexes, so consider them as "look-up".

For one FK to one PK:

ALTER TABLE table1
   ADD FOREIGN KEY (fk1)
     REFERENCES table2 (key1) ON DELETE RESTRICT

If a key on table 2 is composite (key1, key2)

ALTER TABLE table1
   ADD FOREIGN KEY (fk1,fk2)
     REFERENCES table2 (key1,key2) ON DELETE RESTRICT

Answer 3

Try adding 3 constraints: 2 nullable foreign keys for field1 and field2, and a constraint that only one of two is not null.

Of course, you can relax the constraints and omit the last one.