Re: unique constraint with significant nulls?

[Mike Blackwell <mike.blackwell@xxxxxxx>]

Ah.  A pair of constraints.  I see.

Thanks!

__________________________________________________________________________________
Mike Blackwell | Technical Analyst, Distribution Services/Rollout Management | RR Donnelley
1750 Wallace Ave | St Charles, IL 60174-3401
Office: 630.313.7818
Mike.Blackwell@xxxxxxx
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On Tue, Sep 25, 2012 at 12:37 PM, Andreas Joseph Krogh <andreak@xxxxxxxxxxxx> wrote:

On 09/25/2012 05:05 PM, Mike Blackwell wrote:

How would one go about building a multi-column unique constraint where null is a significant value, eg. (1, NULL) <> (2, NULL)?

I see a number of references to not being able to use an index for this, but no mention of an alternative.  Any pointers would be appreciated

create table my_table(
some_column varchar not null,
other_column varchar);

create unique index my_idx on my_table(some_column, other_column) where other_column is not null;
create unique index my_fish_idx on my_table(some_column) where other_column is null;

insert into my_table (some_column, other_column) values('a', 'a');
insert into my_table (some_column, other_column) values('a', 'b');
insert into my_table (some_column) values('a');
insert into my_table (some_column) values('b');

-- fails
insert into my_table (some_column, other_column) values('a', 'a');
-- also fails
insert into my_table (some_column) values('a');

result:

andreak=# insert into my_table (some_column, other_column) values('a', 'a');
ERROR:  duplicate key value violates unique constraint "my_idx"
DETAIL:  Key (some_column, other_column)=(a, a) already exists.

andreak=# insert into my_table (some_column) values('a');
ERROR:  duplicate key value violates unique constraint "my_fish_idx"
DETAIL:  Key (some_column)=(a) already exists.


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Andreas Joseph Krogh<andreak@xxxxxxxxxxxx>  - mob: +47 909 56 963
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