ANSI Options Part 1 - ANSI_NULLS
This article continues my series on ANSI settings in SQL Server. The other parts are:
SQL Server comforms to a number of ANSI standards to varying degrees. The level of compliance or even whether a particular object behaves as per an ANSI Standard is usually goverened by an option setting. In this series I plan to look at the various options that are available in SQL Server. This article looks at ANSI_NULLS.
What are ANSI_NULLS
The NULLability of columns is an oft debated topic; whether to use them or not is something that will probably always be argued. How the server treats NULLs, however, is something that is not argued. But it is something that you should be aware of.
The primary effect of this setting is to control how the server handles the comparison operations of equals (=) and not equals (<>) when dealing with null values. SQL Server may either conform to the ANSI SQL-92 standard (ON) or not (OFF).
The SQL-92 standard specifies that any equals or not equals comparison against a NULL value should return a false, or no rows. This would mean that even if a table contains a NULL value, no rows are returned. An example may help clarify this:
Suppose I have this table and run the select statement below it:
create table MyTable(MyID int, MyChar char(1)) go insert MyTable select 1, 'A' insert MyTable select 2, 'B' insert MyTable select 3, NULL insert MyTable select 4, NULL select * from MyTable MyID MyChar --------- ------------- 1 A 2 B 3 NULL 4 NULL select * from MyTable a where a.MyChar = NULL
With ANSI_NULLS set to ON, this should return 0 rows. And if you run a "SET ANSI_NULLS ON" before running the select statement, you will find that no rows are returned.
SQL Server, however, gives you the option to change this behavior. If you set this option to OFF, then if there are NULL values, they will be returned.
SET ANSI_NULLS OFF select * from MyTable where MyChar = NULL MyID MyChar --------- ------------- 3 NULL 4 NULL
What to do?
The default setting is ANSI_NULLS depends on how you connect to SQL Server. The SQL Server ODBC driver and OLEDB providers set this option to ON. Books online recommends that connections set this option to OFF, but this is for older applications and behaviors. For the most part, unless you have a compelling reason not to, assume and set this option to ON. Most of your clients will depend on the defaults.
There are a few places where you must set this or consider it. For stored procedures, the setting at the time of compilation, not execution, is the one that applies. Again, compile your stored procedures with this set to ON. If you work with indexes on computed columns or views, then this must be set to ON. For distributed queries, this must be set to ON.
- Books Online - Search "Set ANSI_NULLS"
- Online BOL
Nothing earth shattering in this article. Indeed, most of this information can be found in Books Online. However, it is an area that relates to questions I see posted in various forums. Understanding the workings of SQL Server is essential to developing stable and robust applications.
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