db2 - SQL Constraint that one column value cannot be greater than another in a different table -

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this isn't nice question blending business logic db structure, not decision so:

is possible define constraint infers value of 1 column (table a, column x) cannot greater value of (table b, column y) referenced via foreign key:

table_a     id (primary key)     x (int value) table_b     a_id (foreign key table_a)     y (int value)

i.e. want enforce values of y, y < l l value x table_b.a_id == table_a.id

i'm working db2.

is possible define constraint infers value of 1 column (table a, column x) cannot greater value of (table b, column y) referenced via foreign key:

no. require using select statement within check constraint, , db2 doesn't support that. if did support using select statement way, this.

alter table_b add constraint table_b_y_lt_l check (y < (select x table_a table_a.id = table_b.a_id));

the select statement return single value, because table_a.id unique. but, said, db2 doesn't support select statements in check constraints. don't think current dbms does.

workarounds

there couple of workarounds. first, write trigger. second, store column "x" in both tables, , use foreign key constraint , check constraint implement requirement.

-- since "id" unique, combination of "id" , "x" unique.  -- declaring "unique (id, x)" lets these 2 columns target of  -- foreign key reference. -- create table table_a (     id integer primary key,     x integer not null,     unique (id, x) );  -- foreign key references both columns in "table_a". check constraint -- indirectly guarantees 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 standard sql. should work in current sql dbms.


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