Thread: [SQL 2000]Composite key (combination of PK from 2 tables) but allow null in the 2nd c

Here's an example of what I was talking about. Note that I set "Enforce Foreign Key Constraint" to False for the relationship between Table2 and Table3 in order to allow NULLs in the Table2ID column.

To illustrate it better here is a sample data: Column1 Column2 R1 NULL R2 NULL R2 X1 R2 X2 R3 NULL As you can see the combined columns is unique, that is the requirement, is there a workaround that I can make a Composite Key out of those two tables? Another thing, those two columns are PK from other tables, with the first column there is no problem cascading updates, but not so with the 2nd column, aside from a trigger is there any other trick...

PKs can not contain NULL values, so Jmcilhinney is correct what you want to do is not possible. Not leaving a null in the second column of the key.