How can I achieve a unique constraint with two fields?

Question

I have a table with e.g. Name and IsDeleted fields. I want to add a row constraint so that only one Name value can have IsDeleted as 'false'. There can be many duplicate Name values, but they must all have IsDeleted asd true.

How would I write this check constraint ?

Answer 1

Assuming the following table definition

 CREATE TABLE T
 (
 Name VARCHAR(50) NOT NULL,
 IsDeleted BIT NOT NULL
 )

Then you can achieve this with a unique index filtered to only include those Names that you wish to apply the constraint on.

CREATE UNIQUE INDEX ix ON T(Name) WHERE IsDeleted = 'false'