UNIQUE and OR in one SQL Server constraint?

Question

I have a table where I want a constraint that checks the following (letters are column names, W is a 'bit' datatype):

If and only if 'W' = 0, check that X, Y & Z are unique.
Else, allow all new data.

I can do the 'W' is False and the Check X, Y & Z are unique separately, but merging the logic to allow for the above is what I can't figure out.

I tried

W = 1 OR UNIQUE (X, Y, Z)

but SSMS is saying that it's invalid.

Help is appreciated in advance :)

Answer 1

You cannot do that through a constraint. Instead, use a filtered unique index:

create unique index unq_x_w on t(x) where w = 0;
create unique index unq_y_w on t(y) where w = 0;
create unique index unq_z_w on t(z) where w = 0;

Actually, the above interprets your question as each column is unique. If you want the triple to be unique:

create unique index unq_xyz_w on t(x, y, z) where w = 0;