Re: Question: unique on multiple columns

On Wed, 2003-02-05 at 12:15, Richard Huxton wrote:
> On Wednesday 05 Feb 2003 2:47 pm, roverr wrote:
> > On Wed, 2003-02-05 at 09:09, will trillich wrote:
> > > you can
> > >
> > >     create table something (
> > >         a int4,
> > >         b varchar(20),
> > >         c timestamp
> > >     );
> > >     create unique index on something ( a, c );
> > >     create unique index on something ( b, c, a );
> > >
> > > i don't understand your cols 4-9, tho. is this what you're
> > > looking for?
> >
> > Yes, thanks, thats what I was looking for.
> > Columns 4-9 are data that that corresponds to a unique
> > combination of b and c (and necessarily a).
> > Regards, Gary
>
> Note that a unique index on (a,c) necessarily implies unique combinations of
> (a,c,b) - since you can only have one (a,c) pair, there can only be one value
> for "b".
>
> In the case you described it looks like you have a redundant key.
> > > col 1: id, type serial, primary key
> > > col 2: host_id, type integer, foreign key to hosts table
> > > col 3: data_time, type timestamp
> > > col 4 - 9 data that is unique to col 2 and 3
>
> If col1=a,col2=b,col3=c you have unique(a), unique(b,c) if I understand what
> you're saying. You could drop "a" altogether and just use (b,c) as your
> primary key (since that key means something, unlike the serial).

Thank you Richard.  I like your suggestion, I can do away
with an index and make it easy to reference the table with
an integer.
You've correctly determined and answered the
question I should've asked.

Regards, Gary

>
> If you reference this table a lot, you might want to keep "a" so you can refer
> to an integer rather than (varchar,timestamp).
>
> --
>   Richard Huxton