Is it possible to define a constraint the infers the value of one column (Table A, Column X) cannot be greater than the value of another (Table B, Column Y) referenced via a foreign key:
No. That would require using a SELECT statement within a CHECK constraint, and DB2 doesn't support that. If it did support using a SELECT statement that way, it would look something like this.
ALTER TABLE_B
ADD CONSTRAINT TABLE_B_Y_LT_L
CHECK (Y < (SELECT X FROM TABLE_A WHERE TABLE_A.ID = TABLE_B.A_ID));
The SELECT statement would return a single value, because TABLE_A.ID is unique. But, like I said, DB2 doesn't support SELECT statements in check constraints. I don't think any current dbms does.
Workarounds
There are a couple of workarounds. First, you could write a trigger. Second, you could store column "X" in both tables, and use a foreign key constraint and a check constraint to implement your requirement.
-- Since "id" is unique, the combination of "id" and "x" is also unique.
-- Declaring "unique (id, x)" lets these two columns be the target of a
-- foreign key reference.
--
create table table_a (
id integer primary key,
x integer not null,
unique (id, x)
);
-- The foreign key references both columns in "table_a". The check constraint
-- indirectly guarantees that y < table_a.x.
--
create table table_b (
a_id integer not null,
a_x integer not null,
y integer not null,
primary key (a_id, a_x, y),
foreign key (a_id, a_x) references table_a (id, x),
check (y < a_x)
);
This is standard SQL. It should work in any current SQL dbms.