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Useful Functions

Nz( variant, [ value_if_null ] )

Return defined value if variant is null. value_if_null is optional. If value_if_null is not defined, then it will return zero-length string.
Example of usage in MS Access Query Design View:
Field: Expr1: Nz(AnyColumValue, "Value is null")

Ask for user input in query

You can use square bracket to ask for user input each time that the query is ran.

SELECT TableA.col
FROM TableA
WHERE TableA.col=[Enter a value];

IIF( condition, value_if_true, value_if_false )

If condition is true, then return value_if_true. Otherwise, return value_if_false.
Example: IIF([Amount]>100, "Large", "Small")

References
http://www.filex.com/htmlhelp/traxhelpMS_Access_Functions.html