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 particul

Answer 1

isnull(name,'') <> name

Well I can see them using this because this way if the name doesn't match or is null it returns as a failed comparison. This really means: name is null or name <> name

Where as this one name is not null just checks to see if the name is null.

Answer 2

These two queries are not the same. For example, I do not have a middle name, this is a known fact, which can be stored as

MiddleName=''

However, if we don't know someone's middle name, we can store NULL. So, ISNULL(MiddleName, '') means "persons without known middle names".

Answer 3

is not null

Will only check if the field is not null. If the field contains an empty string, then the field is no longer null.

isnull(name, '') <> name

Checks for both a null and an empty string.

Answer 4

I apparently misread your question. So let me strike my first answer and try this one:

isnull(name,'') <> ''

is a misguided shortcut for

name is not null and name <> ''

Answer 5

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.

Answer 6

isnull(name,'') <> :name is shorthand for (name is null or name <> :name) (assuming that :name never contains the empty string, thus why shorthands like this can be bad).

Performance-wise, it depends. or statements in where clauses can give extremely bad performance. However, functions on columns impair index usage. As usual: profile.