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I am using SQL Server 2005. I want to constrain the values in a column to be unique, while allowing NULLS.
My current solution involves a unique index on a view like so:
CREATE VIEW vw_unq WITH SCHEMABINDING AS SELECT Column1 FROM MyTable WHERE Column1 IS NOT NULL CREATE UNIQUE CLUSTERED INDEX unq_idx ON vw_unq (Column1) Any better ideas?
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PostgreSQL treats NULL as distinct value, therefore, you can have multiple NULL values in a column with a UNIQUE index . When you define a primary key or a unique constraint for a table, PostgreSQL automatically creates a corresponding UNIQUE index.
Although null values represent unknown values, when it comes to indexing, a null value is treated as being equal to other null values. Therefore, if a unique index consists of a single column, only one null value is allowed-more than one null value would violate the unique constraint.
Key Differences Between Primary key and Unique key: Primary key will not accept NULL values whereas Unique key can accept NULL values. A table can have only one primary key whereas there can be multiple unique key on a table.
Using SQL Server 2008, you can create a filtered index: http://msdn.microsoft.com/en-us/library/cc280372.aspx. (I see Simon added this as a comment, but thought it deserved its own answer as the comment is easily missed.)
Another option is a trigger to check uniqueness, but this could affect performance.
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The calculated column trick is widely known as a "nullbuster"; my notes credit Steve Kass:
CREATE TABLE dupNulls ( pk int identity(1,1) primary key, X int NULL, nullbuster as (case when X is null then pk else 0 end), CONSTRAINT dupNulls_uqX UNIQUE (X,nullbuster) ) If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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