Обсуждение: Unique constraint over null values

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

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 BerkusAglio Database SolutionsSan Francisco

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?
> 

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 BerkusAglio Database SolutionsSan Francisco