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by Jiří {x2} Činčura

Unique constraint on MS SQL?!

26 Sep 2009 2 mins Databases in general, Firebird, MS SQL Server, Oracle Database

I have a strong feeling that MS SQL knows I don’t like it. I don’t say it’s a bad database, but sometimes it’s really “interesting”. 😉

OK, so what I faced this time? Let’s start with basic facts. The NULL value in database is a special value. NULL and NULL is again NULL, NULL and False is again NULL and NULL isn’t equal to NULL. Two days ago I was creating a chain of records in database (which isn’t too relationalish, but …) and I needed unique constraint on one column. No problem you may think. So did I. But not in MS SQL.

Check this example:

1> create table test(id int primary key, foo int);
2> create unique index idx_text on test(foo);
3> go
1> insert into test values (1, null);
2> go
(1 rows affected)
1> insert into test values (2, null);
2> go
Msg 2601, Level 14, State 1, Server X2-001SQLEXPRESS, Line 1 Cannot insert duplicate key row in object 'dbo.test' with unique index 'idx_text '.
The statement has been terminated.

What a mess! Looks like somebody in MS SQL engine team thinks NULL == NULL. After some railing, testing on Firebird and asking my friend about Oracle, I googled: Looks like I’m not alone; feeling better. 😉

Luckily I’m working on MS SQL 2008 and I can use filtered indices. Created unique index with where clause where <column> is not null and I’m done. But boy that was a bitter finding. Hope I’ll not be punched to the face with these basics again in an at least two months.

Profile Picture Jiří Činčura is .NET, C# and Firebird expert. He focuses on data and business layers, language constructs, parallelism, databases and performance. For almost two decades he contributes to open-source, i.e. FirebirdClient. He works as a senior software engineer for Microsoft. Frequent speaker and blogger at