I have a table with two columns whose combined value I want to be unique:
create table Foo (
A integer,
B integer,
primary key(A),
unique(A,B)
);
This works fine except when B is null, when I can have multiple rows containing
identical values of the form (A,null).
Is there an easy way to constrain the values of A to be unique even when B is
null? (I could try to change things so that empty strings are used instead of
nulls, but that would involve changing existing code and it will take quite a
bit of work to ensure that there aren't any unexpected knock-on effects, so I
prefer to stick with nulls if I can.)
TIA,
--
John English
