From: Peter Brawley
Date: March 30 2005 5:25am
Subject: Re: Regarding NULL and '' (null string) treatment in MYSQL
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Narasimha,

In programming languages, a 'null string' is empty, but in ANSI SQL, 
NULL means unknown, _not_ empty, so
in ANSI SQL, NULLs are never equal to anything, not even themselves: the 
expressions NULL=NULL, NULL<>NULL and NULL='' all evaluate to NULL.

Microsoft SQL has an 'ansi_nulls' setting which turns off this feature. 
With ansi_nulls turned off, NULL works as you want it to (thus breaking 
a lot of traditional SQL code). Perhaps Oracle also has such a setting 
too. MySQL doesn't.

Referential integrity (RI) is meant to work oppositely to your 
description: absent a matching key value in the parent table, a child 
row cannot be inserted. What you describe looks like what's often called 
the 'zeroth row' workaround: create a parent row with an empty key 
value, then add matching child rows. It wrecks RI. If you absolutely 
must add child rows before adding the parent row, use empty values, not 
NULLs, but most DBAs would insist that you to revise the design such 
that empty parent key values are not permitted.

Peter Brawley
http://www.artfulsoftware.com

-----

lakshmi.narasimharao@stripped wrote:

>Hi,
>
>    MySQL treats NULL, '' (empty string) as different.
>
>
>    I mean when I select from/insert into a table, its behaviour is
>different. 
>
>
>   Select * from table1 where name=''; 
>
>
>   is different from
>
>
>   Select * from table1 where name=NULL;  
>
>
>   Similarly
>
>   Insert into table1(name) values('')  is different from
>
>
>   Insert into table1(name) values(NULL) 
>
>
>   Please suggest me here, how to overcome this '' (null string)
>problem.
>
> Scenario:
>
>
> I have two tables, one is parent and one is child.
>
> Child is referencing 3 fields in parent table.
>
>Parent table records are empty. Now I am trying to insert into the child
>table with  '' (null string) values into these 3 fields (which are
>referencing to the parent table).  Actually insert into the child table
>in this scenario should be successful, but I am not able to make it
>success because of the problem described above ( Treatment of NULL, ''
>are different).
>
>Note: If the parent is not having any records, we can insert into the
>child. BCS, in this case referential integrity won't work.
>
>  
>
>Ps: In oracle, it is not the case it treats NULL,'' as same.
>
>
>Please suggest me how to proceed here.
>
>
>Thanks,
>Narasimha
>
>
>
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