sql - isnull vs is null

Question

I have noticed a number of queries at work and on SO are using limitations in the form:

isnull(name,'') <> ''

Is there a particular reason why people do that and not the more terse

name is not null

Is it a legacy or a performance issue?

Answer 1

where isnull(name,'') <> ''

is equivalent to

where name is not null and name <> '' 

which in turn is equivalent to

where name <> ''

(if name IS NULL that final expression would evaluate to unknown and the row not returned)

The use of the ISNULL pattern will result in a scan and is less efficient as can be seen in the below test.

SELECT ca.[name],
       [number],
       [type],
       [low],
       [high],
       [status]
INTO   TestTable
FROM   [master].[dbo].[spt_values]
       CROSS APPLY (SELECT [name]
                    UNION ALL
                    SELECT ''
                    UNION ALL
                    SELECT NULL) ca 


CREATE NONCLUSTERED INDEX IX_TestTable ON dbo.TestTable(name)

GO


SELECT name FROM TestTable WHERE isnull(name,'') <> ''

SELECT name FROM TestTable WHERE name is not null and name <> ''
/*Can be simplified to just WHERE name <> '' */

Which should give you the execution plan you need.