Much Ado about Nothing: Interesting Facts about Null

See: http://www.itjungle.com/fhg/fhg091405-story01.html
It's hard to believe that this much could be said about things that don't exist.

See also: http://www.itjungle.com/mpo/mpo082803-story02.html
The NULL Nemesis

The concept is simple enough. The ShipDate for the given OrderID is placed in program variable RPGShipDate. But what if ShipDate contains a NULL in the file? The way the program is coded now, when the statement retrieves a NULL the SQLCOD will contain error code -305, which has the primary error description of "indicator variable required."

This message indicates that SQL can't return the NULL value into a HLL program variable without the assistance of an "indicator" variable. An indicator variable is simply an extra variable, defined with an SQL data type of SMALLINT, containing either a zero or a negative 1, to indicate if the given variable is NOT NULL or NULL, respectively. Here's the revised code using an indicator variable that will behave correctly if a NULL is read:

D ShipDateNull    S              5I 0
D NULL            S              5I 0 Inz(-1)
C                   Eval      OrderID=11074
C/EXEC SQL
C+  SELECT ShipDate
C+    INTO :RPGShipDate:ShipDateNull
C+    FROM Orders
C+   WHERE OrderID=:OrderID
C/END-EXEC
C                   Select
C                   When      SQLCOD=*Zero
C                   If        ShipDateNull=NULL
  … ShipDate is NULL – Process accordingly
C                   Else
  … ShipDate is NOT NULL – Process accordingly
C                   EndIf