Constraint

Question

Hi there.
We have to tables AA (colA1, colA2) and BB (colB1, colB2).
Table AA is parent to BB.
BB.colB1 has a FK reference constraint to AA.colA1.
The problem is that the relation is a many-to-many, so a value in BB.colB1 can actually be inserted if the referencing value just exists (one or more times) in AA.colA1. The constraint I have used so far doesn't permit that (constraint fk_gasforecast_enhplads references op_kvvenergianlaeg(enhedsplads) not null).
How do I create a reference constraint with a many-to-many relation?
Thanks
(sorry for my limited imagination, mrs. Garcia !)
chr

Answer 1

I'm not sure what your problem is here-- foreign key relationships already allow for the 1-to-many relationship you seem to be describing
  1* create table parent( key NUMBER PRIMARY KEY, value VARCHAR2(100) )
scott@jcave > /
Table created.
Elapsed: 00:00:00.02
scott@jcave > create table child( pk NUMBER PRIMARY KEY,
  2                               fk NUMBER REFERENCES parent( key ),
  3                               value VARCHAR2(100) );
Table created.
Elapsed: 00:00:00.01
scott@jcave > insert into parent( key, value ) values ( 1, 'First parent' );
1 row created.
Elapsed: 00:00:00.00
scott@jcave > ed
Wrote file afiedt.buf
  1* insert into parent( key, value ) values ( 2, 'Second parent' )
scott@jcave > /
1 row created.
Elapsed: 00:00:00.00
scott@jcave > insert into child( pk, fk, value ) values ( 1000, 1, 'First child of first parent'
1 row created.
Elapsed: 00:00:00.01
scott@jcave > ed
Wrote file afiedt.buf
  1* insert into child( pk, fk, value ) values ( 1001, 1, 'Second child of first parent' )
scott@jcave > /
1 row created.
Elapsed: 00:00:00.00
scott@jcave > ed
Wrote file afiedt.buf
  1* insert into child( pk, fk, value ) values ( 1002, 2, 'First child of second parent' )
scott@jcave > /
1 row created.
Elapsed: 00:00:00.00
scott@jcave > ed
Wrote file afiedt.buf
  1* insert into child( pk, fk, value ) values ( 1003, 2, 'Second child of second parent' )
scott@jcave > /
1 row created.
Elapsed: 00:00:00.00If you need many-to-many relationships, you need to create a mapping table.
Justin
Distributed Database Consulting, Inc.
http://www.ddbcinc.com/askDDBC