Functions and Directives

Function IsNull() Foundation

Tests if an expression evaluates to NULL/NIL.

Syntax 
IsNull( <Expression> ) --> lIsNull
IsNull( <Expression> , <ReplacementValue> ) --> xValue
Parameters
<Expression>
Specifies the expression to be evaluated.
<ReplacementValue>
If <Expression> evaluates to NULL/NIL, <ReplacementValue> is the value returned. <ReplacementValue> must have the same type as <Expression>.
Return

If IsNull() is called with a single parameter, IsNull() returns .T. (true) if an expression evaluates to a NULL/NIL value; otherwise, IsNull( ) returns .F. (false).

If the parameter <ReplacementValue> is passed, the function IsNull() returns the value of <ReplacementValue> if <Expression> evaluates to NULL/NIL. Otherwise the value of <Expression> is returned.

Description

The function IsNull() should be used instead of an explicit comparison with NIL wherever <Expression> is related to database operations. Using IsNull() instead of <Expression> != NIL helps to keep business logic independent of the underlying database management system due to the fact that all major DBMS vendors support IsNull().

The NULL/NIL value indicates that the variable contains no valid data. NULL/NIL is not the same as Empty(). It is also not the same as a zero-length string (""), which sometimes is referred to as a null string.

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