Adding a foreign key constraint

Question

Is it possible to add a constraint just like foreign key after inserting some values in the table?

If it's possible how can I do it, because I'm trying to add it with

alter table ...add constraint ..foreign key .." 

But I get an error:

impossible de valider (SYS.COMPAGNIEID1) - clés parents introuvables.

Answer 1

In order to have a define a foreign key, you have to have defined a primary key in the "parent" table. For example, using the standard EMP and DEPT tables, you would first define the column DEPTNO as a primary key in the DEPT table. You would then define DEPTNO in EMP table as a foreign key, that references DEPTNO in DEPT. If you have data in you table, (by default) the data will be validated to make sure the data conforms to the constraints.