RE: Useful Oracle books - C.J. Date theory vs. practicality

Lex, 
 
How about using: 'string' IS NULL 
    instead of 
'string' = NULL?
 
 
19:05:15 SQL> set serveroutput on
19:05:22 SQL> @three_way_logic
19:05:32 SQL> begin
19:05:32   2      if     (&&condition) then
19:05:32   3             dbms_output.put_line('THEN branch');
19:05:32   4      else
19:05:32   5             dbms_output.put_line('ELSE branch');
19:05:32   6      end    if;
19:05:32   7      if not (&condition) then
19:05:32   8             dbms_output.put_line('THEN branch');
19:05:32   9      else
19:05:32  10             dbms_output.put_line('ELSE branch');
19:05:32  11      end if;
19:05:32  12  end;
19:05:32  13  /
Enter value for condition: 'Mort subite' is NULL
ELSE branch
THEN branch
 
PL/SQL procedure successfully completed.
 
Elapsed: 00:00:00.06
19:05:54 SQL>

 
________________________________

From: oracle-l-bounce@xxxxxxxxxxxxx on behalf of Lex de Haan
Sent: Fri 5/28/2004 3:56 PM
To: oracle-l@xxxxxxxxxxxxx
Subject: RE: Useful Oracle books - C.J. Date theory vs. practicality


Jared, I stored your example in a script, making it prompt for a condition:
 
SQL*Plus: Release 10.1.0.2.0 - Production on Fri May 28 21:51:24 2004
Copyright (c) 1982, 2004, Oracle.  All rights reserved.
 
SQL> @jared
Enter value for condition: 'Mort subite' = NULL
ELSE branch
ELSE branch
 
PL/SQL procedure successfully completed.
 
SQL> @jared
Enter value for condition: 1=1
THEN branch
ELSE branch
 
PL/SQL procedure successfully completed.
 
SQL> list
  1  begin
  2    if     (&&condition) then
  3           dbms_output.put_line('THEN branch');
  4    else
  5           dbms_output.put_line('ELSE branch');
  6    end    if;
  7    if not (&condition) then
  8           dbms_output.put_line('THEN branch');
  9    else
 10           dbms_output.put_line('ELSE branch');
 11    end if;
 12* end;
SQL> 
 
Kind regards,
Lex.
 
---------------------------------------------
visit my website at http://www.naturaljoin.nl <http://www.naturaljoin.nl/> 
---------------------------------------------
 
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]On 
Behalf Of Powell, Mark D
Sent: Friday, May 28, 2004 20:48
To: 'oracle-l@xxxxxxxxxxxxx'
Subject: RE: Useful Oracle books - C.J. Date theory vs. practicality


I think the point was if part of some condition was Null then the statement is 
not true, but is not false either as technically the result is unknown so in 
pl/sql you potentially should have a construct that looks like
 
If true 
   ...
Else if false
  ...
Else [it is null so]
 
I have seen more than one bug traced to the failure of a developer to remember 
that in pl/sql just because something is not true that does not mean it is 
false.  Failure to remember this results in attempts to find rows for the FALSE 
condition when there is no value to be used to query for those rows. 
 
In native COBOL there is only True and False.  However if you use COBOL with an 
RDBMS both Oracle and DB2 provide a null value indicator feature that can be 
used to allow program logic to properly handle the data.  No matter what 
language is used when the data comes from an RDBMS that supports NULLs this 
issue of TRUE/ FALSE/ NULL probably exists.
 
Just adding to the noise level.
-- Mark D Powell --

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