if Function

The if function returns one value if a test evaluates to TRUE and a different value if the test evaluates to FALSE.

Syntax
if(test, true_value, false_value)

test is a value or expression that evaluates to TRUE or FALSE.

true_value is the value that is returned when test is TRUE.

false_value is the value that is returned when test is FALSE.

Examples
if(not IsNull(SoldQuantity), SoldQuantity, OutputQuantity)