problem with inserting a generated seq no into not null field

Поиск
Список
Период
Сортировка
Искать

problem with inserting a generated seq no into not null field

От:
mike <mike@bristolreccc.co.uk>
Дата:
I have the following generic query which I am trying to use to insert
into a 3 field table

INSERT INTO tb_files (subj, area, file_no) select '4', '8',(SELECT CASE
WHEN (4||8) NOT IN (SELECT (subj||area) FROM tb_files) THEN '1' ELSE
max(file_no)+1 END FROM tb_files GROUP BY (subj||area),subj,area HAVING
subj = '4' AND area = '8');

However the first entry will not go in with a 

 'null value in column "file_no" violates not-null constraint'

message

Anyone any ideas on this - the CASE statement does not seem to be being
evaluated

Re: problem with inserting a generated seq no into not

От:
Stephan Szabo <sszabo@megazone.bigpanda.com>
Дата:

> I have the following generic query which I am trying to use to insert
> into a 3 field table
>
> INSERT INTO tb_files (subj, area, file_no) select '4', '8',(SELECT CASE
> WHEN (4||8) NOT IN (SELECT (subj||area) FROM tb_files) THEN '1' ELSE

That's not really a good way to detect the existance of a row with those
values.  If either subj or area is allowed to be NULL, wierdness occurs
(*). Even if neither is allowed to be null, if the values can ever have
more than one digit wierdness occurs.

(*) - IIRC, even if no row exists with (4,8) as (subj,area), if there's a
row where subj || area is NULL, you will never get true from the NOT IN
clause and the second alternative will always be chosen. NULLs and IN
don't play very well together.

Re: problem with inserting a generated seq no into not null field

От:
Manfred Koizar <mkoi-pg@aon.at>
Дата:
On Tue, 08 Jun 2004 12:37:48 +0100, mike 
wrote:
>INSERT INTO tb_files (subj, area, file_no) select '4', '8',(SELECT CASE
>WHEN (4||8) NOT IN (SELECT (subj||area) FROM tb_files) THEN '1' ELSE
>max(file_no)+1 END FROM tb_files GROUP BY (subj||area),subj,area HAVING
>subj = '4' AND area = '8');

If there is no row with subj = '4' AND area = '8', then SELECT ...
HAVING returns no rows and therefore max(file_no) is NULL.  And NULL+1
is NULL.  Try

	INSERT INTO tb_files (subj, area, file_no)
	SELECT '4', '8', coalesce(max(file_no), 0)+1
	  FROM tb_files
	 WHERE subj = '4' AND area = '8';

Servus
 Manfred
FAQ