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Adding null values to a variable can produce different results based on a SQL Server setting. Look at the statement, "Set concat_null_yields_null off" in the code below.
This tells SQL Server to ignore the null value and display the sum of the non-null variables as the result. Thus, the result in this example will be 'hello.'
If you change the code to use
set concat_null_yields_null on
Then the result will always be null since SQL Server will now no longer ignore the null variable and treat anything added to null as NULL.
Code: declare @var1 varchar(10) declare @var2 varchar(10) set concat_null_yields_null off set @var1='hello' set @var2=null print @var1+@var2
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This was first published in June 2005