Question

I’ve a situation where I want a table containing three fields, an ID, an Building name and a Room name. The ID is just an autoincrement and Building is Not NULL. Room however can be NULL, but I’d like to arrange for the combination of Building and Room to be Unique. Is this possible and how would I set this up?

Thanks,
James

Answer 1

Create a table with a unique constraint:

create table t (id integer primary key,
                building text not null,
                room text,
                unique (building, room));

Insert data:

insert into t (building, room) values ("B1", "R1");
insert into t (building, room) values ("B1", "R2");
insert into t (building, room) values ("B1", null);
insert into t (building, room) values ("B1", "R3");
insert into t (building, room) values ("B1", "R1");
-- Error: columns building, room are not unique
insert into t (building, room) values ("B1", null);
-- Note: This last insert does not violate the constraint!

View the data:

select * from t;
1|B1|R1
2|B1|R2
3|B1|
4|B1|R3
5|B1|