Chris,

From the SQL Server Books Online:
IS [NOT] NULL
Determines whether or not a given expression is NULL.

Syntax
expression IS [ NOT ] NULL

Arguments
expression

Is any valid Microsoft® SQL ServerT expression.

NOT

Specifies that the Boolean result be negated. The predicate reverses its
return values, returning TRUE if the value is not NULL, and FALSE if the
value is NULL.

Result Types
Boolean

Return Code Values
If the value of expression is NULL, IS NULL returns TRUE; otherwise, it
returns FALSE.

If the value of expression is NULL, IS NOT NULL returns FALSE; otherwise, it
returns TRUE.

Remarks
To determine if an expression is NULL, use IS NULL or IS NOT NULL rather
than comparison operators (such as = or !=). Comparison operators return
UNKNOWN if either or both arguments are NULL.

Examples
This example returns the title number and the advance amount for all books
in which either the advance amount is less than $5,000 or the advance is
unknown (or NULL). Note that the results shown are those returned after
Example C has been executed.

USE pubs
SELECT title_id, advance
FROM titles
WHERE advance < $5000 OR advance IS NULL
ORDER BY title_id
Here is the result set:

title_id advance
-------- --------------------------
MC2222   0.0000
MC3026   NULL
PC9999   NULL
PS2091   2275.0000
PS3333   2000.0000
PS7777   4000.0000
TC4203   4000.0000

(7 row(s) affected)

Regards,
Graham R Seach
Microsoft Access MVP
Sydney, Australia

Microsoft Access 2003 VBA Programmer's Reference
http://www.wiley.com/WileyCDA/WileyTitle/productCd-0764559036.html

I can't think of any situation in which you would need IsEmpty in SQL,
Chris. The IsEmpty function returns True only when the argument is an
uninitialized Variant. It does not return True when the argument is Null, or
a zero-length string. The following test code may help to illustrate the
point ...

Public Sub TestSub()

   Dim varTest As Variant
   Dim rst As ADODB.Recordset

   Debug.Print "Before assignment"
   Debug.Print "Is variable empty?", IsEmpty(varTest)
   Debug.Print "Is variable null?", IsNull(varTest)

   varTest = Null
   Debug.Print "After assignment"
   Debug.Print "Is variable empty?", IsEmpty(varTest)

   Set rst = New ADODB.Recordset
   With rst
       .ActiveConnection = CurrentProject.Connection
       .Source = "SELECT TestText FROM tblTest WHERE TestText IS NULL"
       .Open
       Debug.Print "Null field"
       Debug.Print "Records found?", (.BOF = False And .EOF = False)
       Debug.Print "Is field null?", IsNull(.Fields("TestText"))
       Debug.Print "Is field empty?", IsEmpty(.Fields("TestText"))
       .Close
       Debug.Print "Zero-length field"
       .Source = "SELECT TestText FROM tblTest WHERE TestText = ''"
       .Open
       Debug.Print "Records found?", (.BOF = False And .EOF = False)
       Debug.Print "Is field empty?", IsEmpty(.Fields("TestText"))
       .Close
   End With

End Sub

Results in Immediate window ...

testsub
Before assignment
Is variable empty?          True
Is variable null?           False
After assignment
Is variable empty?          False
Null field
Records found?              True
Is field null?              True
Is field empty?             False
Zero-length field
Records found?              True
Is field empty?             False