*SQLNULL is a special keyword like *NULL, but with a different meaning: *SQLNULL means the value for the field is unknown, whereas *NULL means the field is empty.
Fields may not be compared to *SQLNULL with operators such as *EQ and *NE, however *IS and *ISNOT can be used.
*SQLNULL becomes the default for fields that allow SQL Null, regardless of field type, unless another default has been specified. *SQLNULL may be explicitly specified as the default for a field if it has the ASQN attribute. For example, in RDML:
Define Field(#B) Type(*CHAR) Length(8) Input_Atr(ASQN) Default(*SQLNULL)
Also See
ASQN (Allow SQL Nulls) attribute
Assignment, Conditions, and Expressions with Fields allowing SQL Null
