With SQL Server I want a unique index on only the non-null values [duplicate]

Answer 1

In SQL Server 2008, you can create a filtered index, like:

CREATE UNIQUE INDEX indexName ON tableName(isbn) WHERE isbn IS NOT NULL

In earlier versions, create a calculated column which is the ISBN when it exists, and the primary key when the ISBN is null:

CREATE TABLE tableName (
    pk int identity(1,1) primary key,
    isbn int NULL,
    nullbuster as (case when isbn is null then pk else 0 end),
    CONSTRAINT dupNulls_uqX UNIQUE (isbn)
)

Copied from SQL Server UNIQUE constraint with duplicate NULLs, so this is in wiki mode :)