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* Unique constraint over null values
@ 2002-09-30 17:22 Thrasher <thrasher@fibers.upc.es>
2002-09-30 17:33 ` Re: Unique constraint over null values Josh Berkus <josh@agliodbs.com>
0 siblings, 1 reply; 4+ messages in thread
From: Thrasher @ 2002-09-30 17:22 UTC (permalink / raw)
To: pgsql-sql
Hi all,
I have a table like
CREATE TABLE a (
type CHAR (1) NOT NULL,
data1 CHAR (16) NOT NULL,
data2 CHAR (16) NULL
);
where type can be 's' for 'single' and 'x' for extended, so s should
mean that there is only the type and data1 field, and x means that all
fields are set.
How can I set a unique constraint like CHECK (type = 's' AND UNIQUE
(type, data1)) OR (type = 'x' AND UNIQUE (type, data1, data2)) ?
The documentation says that 2 NULL values are different, so no unique
constraint can be set over it.
Thanks in advance,
Thrasher
^ permalink raw reply [nested|flat] 4+ messages in thread
* Re: Unique constraint over null values
2002-09-30 17:22 Unique constraint over null values Thrasher <thrasher@fibers.upc.es>
@ 2002-09-30 17:33 ` Josh Berkus <josh@agliodbs.com>
2002-09-30 17:41 ` Re: Unique constraint over null values Thrasher <thrasher@fibers.upc.es>
0 siblings, 1 reply; 4+ messages in thread
From: Josh Berkus @ 2002-09-30 17:33 UTC (permalink / raw)
To: Thrasher <thrasher@fibers.upc.es>; pgsql-sql
Thrasher,
> where type can be 's' for 'single' and 'x' for extended, so s should
> mean that there is only the type and data1 field, and x means that all
> fields are set.
>
> How can I set a unique constraint like CHECK (type = 's' AND UNIQUE
> (type, data1)) OR (type = 'x' AND UNIQUE (type, data1, data2)) ?
>
> The documentation says that 2 NULL values are different, so no unique
> constraint can be set over it.
Simple. Don't use NULLs. NULL means "unknown". You should use a
differnent value, such as a blank string or "N/A" or "000000000000000" to
represent "intentionally left blank".
Then you can set data2 NOT NULL and use a regular UNIQUE constraint.
BTW, you have defined your data columns as CHAR, not VARCHAR. You do know
that this implies that your data columns will be *exactly* 16 characters in
length?
--
-Josh Berkus
Aglio Database Solutions
San Francisco
^ permalink raw reply [nested|flat] 4+ messages in thread
* Re: Unique constraint over null values
2002-09-30 17:22 Unique constraint over null values Thrasher <thrasher@fibers.upc.es>
2002-09-30 17:33 ` Re: Unique constraint over null values Josh Berkus <josh@agliodbs.com>
@ 2002-09-30 17:41 ` Thrasher <thrasher@fibers.upc.es>
2002-09-30 17:50 ` Re: Unique constraint over null values Josh Berkus <josh@agliodbs.com>
0 siblings, 1 reply; 4+ messages in thread
From: Thrasher @ 2002-09-30 17:41 UTC (permalink / raw)
To: pgsql-sql
Hi Josh,
I think I'll follow your advice, as this is the method that I had in my
pocket to use if I had no response. I tried with TRIM and other
functions in the CHECK constraint, but I guess that I cannot use
functions over a check field.
It surprised me, but anyway, I'll do that.
Thanks a lot for your prompt reply
Thrasher
Josh Berkus wrote:
> Thrasher,
>
>
>>where type can be 's' for 'single' and 'x' for extended, so s should
>>mean that there is only the type and data1 field, and x means that all
>>fields are set.
>>
>>How can I set a unique constraint like CHECK (type = 's' AND UNIQUE
>>(type, data1)) OR (type = 'x' AND UNIQUE (type, data1, data2)) ?
>>
>>The documentation says that 2 NULL values are different, so no unique
>>constraint can be set over it.
>
>
> Simple. Don't use NULLs. NULL means "unknown". You should use a
> differnent value, such as a blank string or "N/A" or "000000000000000" to
> represent "intentionally left blank".
>
> Then you can set data2 NOT NULL and use a regular UNIQUE constraint.
>
> BTW, you have defined your data columns as CHAR, not VARCHAR. You do know
> that this implies that your data columns will be *exactly* 16 characters in
> length?
>
^ permalink raw reply [nested|flat] 4+ messages in thread
* Re: Unique constraint over null values
2002-09-30 17:22 Unique constraint over null values Thrasher <thrasher@fibers.upc.es>
2002-09-30 17:33 ` Re: Unique constraint over null values Josh Berkus <josh@agliodbs.com>
2002-09-30 17:41 ` Re: Unique constraint over null values Thrasher <thrasher@fibers.upc.es>
@ 2002-09-30 17:50 ` Josh Berkus <josh@agliodbs.com>
0 siblings, 0 replies; 4+ messages in thread
From: Josh Berkus @ 2002-09-30 17:50 UTC (permalink / raw)
To: Thrasher <thrasher@fibers.upc.es>; +Cc: pgsql-sql
Thrasher,
> I think I'll follow your advice, as this is the method that I had in my
> pocket to use if I had no response. I tried with TRIM and other
> functions in the CHECK constraint, but I guess that I cannot use
> functions over a check field.
>
> It surprised me, but anyway, I'll do that.
>
> Thanks a lot for your prompt reply
No problem. You should also add a second constraint:
CHECK ((type = 'x' AND data2 <> 'blank value') OR (type = 'y' AND data2 =
'blank value'))
To enforce your other criterion.
Keeop in mind that depending on the rest of your data structure, there are
probably 8 different ways to approach this problem. I'd reccomend, in fact,
a quick reading of Pascal's "Practical Issues in Database Management" on
normalization to see the different table structures that might work for you.
--
-Josh Berkus
Aglio Database Solutions
San Francisco
^ permalink raw reply [nested|flat] 4+ messages in thread
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2002-09-30 17:22 Unique constraint over null values Thrasher <thrasher@fibers.upc.es>
2002-09-30 17:33 ` Josh Berkus <josh@agliodbs.com>
2002-09-30 17:41 ` Thrasher <thrasher@fibers.upc.es>
2002-09-30 17:50 ` Josh Berkus <josh@agliodbs.com>
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