Logic problem with exercise 11 in 4gl by example

The text I am working from contains this code segment:

LET st_stock =
   "SELECT stock_num, description, stock.manu_code, manufact.manu_name, ",
   "unit, unit_price FROM stock, manufact"

IF stock_item IS NOT NULL THEN
  LET st_stock = st_stock CLIPPED, " WHERE stock_num = ", stock_item,
   " AND stock.manu_code = manufact.manu_code",
   " ORDER BY 1, 3"
ELSE
  LET st_stock = st_stock CLIPPED,
  " WHERE stock.manu_code = manufact.manu_code",
  " ORDER BY 1, 3"
END IF

PREPARE slct_run FROM st_stock
DECLARE c_stockpop CURSOR FOR slct_run

The purpose of this code is to populate a popup list containing all the stock
items matching the selection criteria.

Here is my problem.  I must assume that this code has been tested by
innumerable people so whatever is happening is due to some misunderstanding on
my part.  However:

The screen field stock_item occurs before the screen field manu_code.  The
popup may be called from either field.  If the stock_item field is NULL, as it
will be if one chooses to display the popup, then the manu_code field will also
necessarily be NULL as it has not yet been activated for input.

But this code seems to say that if the stock_item is NULL then it expects that
the manu_code will not be NULL.  I have DISPLAYed the resulting query in this
case and it says this:

SELECT stock_num, description, stock.manu_code, manufact.manu_name, unit,
unit_price FROM stock, manufact WHERE stock.manu_code = manufact.manu_code
ORDER BY 1, 3


But this will always return the empty set since there cannot be any stock_items
without an associated manu_code.

What am I missing?


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James B. Byrne                mailto:[email protected]
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