How to use SQL Select to Locate NULL Values

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If a table contains null values (represented as .NULL.) the following SELECT statement will always return zero records even when cFld1 contains null values:

   SELECT * FROM mytable WHERE cFld1 = .NULL.
				

Null should be used when a value is missing, irrelevant, or unknown. When any conditional expression encounters a null value, a .NULL. will be returned. A value of true (.T.) or false (.F.) cannot be returned if part of the expression is unknown or null. The expression in the above example, cFld1 = .NULL., will never evaluate to True, instead it will evaluate to .NULL. and as a result no records will be returned.

To locate .NULL. values in cfld1, use the new IS NULL clause of the SQL SELECT command. The IS NULL clause is new to Visual FoxPro version 3.0. It provides a mechanism for comparing fields with .NULL. values and returning true or false. For example, consider this SELECT command:

   SELECT * FROM mytable WHERE cfld1 IS NULL
				

In this example, whenever cfld1 is .NULL., the IS NULL clause will cause the SELECT command to return a .T. and the desired records.