Thread: Unique Constraint with foreign Key

Unique Constraint with foreign Key

From
mohan@physics.gmu.edu
Date:
Hi all i am using postgres and torque
I have a column x in Table A of type varchar and NOT NULL. I want to
impose a unique key constraint on that table and column y is the primary
key.
I have another Table B with column x. can i make this column x as a
foreign key on column x in table A.
or Can i have varchar types  of size 50 as primary keys in Postgres.

please let me know

--Mohan




Re: Unique Constraint with foreign Key

From
Bruno Wolff III
Date:
On Tue, Feb 10, 2004 at 12:03:36 -0500, mohan@physics.gmu.edu wrote:
> Hi all i am using postgres and torque
> I have a column x in Table A of type varchar and NOT NULL. I want to
> impose a unique key constraint on that table and column y is the primary
> key.
> I have another Table B with column x. can i make this column x as a
> foreign key on column x in table A.

Yes. The reference will need to specify column x, since the primary key
column (y) will be the default.

> or Can i have varchar types  of size 50 as primary keys in Postgres.

Yes. But unless the 50 character limit comes from a business rule, you
might want to use the type 'text'.


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From
Mona
Date:
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Re: Unique Constraint with foreign Key

From
"Greg Patnude"
Date:
Pleas also note that the referenced column in the foreign table either needs
to be the PRIMARY KEY or have a unique constraint on it or maybe it just
requires an index on it -- I'm not sure but I discovered that if the column
in the foreign table (containing the REFERENCED key...) is NOT the primary
key column -- the REFERENCES a(x) will faill unless a.x is specified as
'UNIQUE' -- as in the following example:

create table a (
y integer not null primary key default nextval('nexta_seq'),
x varchar not null UNIQUE

);

create table b (
   z integer not null PRIMARY KEY default nextval('nextbz_seq'),   x varchar NOT NULL REFERENCES a(x),

);


-- 
Greg Patnude / The Digital Demention
2916 East Upper Hayden Lake Road
Hayden Lake, ID 83835
(208) 762-0762

"Bruno Wolff III" <bruno@wolff.to> wrote in message
news:20040210201521.GA13737@wolff.to...
> On Tue, Feb 10, 2004 at 12:03:36 -0500,
>   mohan@physics.gmu.edu wrote:
> > Hi all i am using postgres and torque
> > I have a column x in Table A of type varchar and NOT NULL. I want to
> > impose a unique key constraint on that table and column y is the primary
> > key.
> > I have another Table B with column x. can i make this column x as a
> > foreign key on column x in table A.
>
> Yes. The reference will need to specify column x, since the primary key
> column (y) will be the default.
>
> > or Can i have varchar types  of size 50 as primary keys in Postgres.
>
> Yes. But unless the 50 character limit comes from a business rule, you
> might want to use the type 'text'.
>
> ---------------------------(end of broadcast)---------------------------
> TIP 7: don't forget to increase your free space map settings
>




Re: Unique Constraint with foreign Key

From
Jan Wieck
Date:
Greg Patnude wrote:
> Pleas also note that the referenced column in the foreign table either needs
> to be the PRIMARY KEY or have a unique constraint on it or maybe it just
> requires an index on it -- I'm not sure but I discovered that if the column
> in the foreign table (containing the REFERENCED key...) is NOT the primary
> key column -- the REFERENCES a(x) will faill unless a.x is specified as
> 'UNIQUE' -- as in the following example:

This is according to the SQL specification, which doesn't like doubts.

Imagine rows (1, 99), (2, 99) in table a and row (3, 99) in table b. 
Which of the a-rows is now referenced and am I allowed to delete the 
other? There are good arguments either way, but if you require a UNIQUE 
on a.x, then this question will never come up.


Jan

> 
> create table a (
> y integer not null primary key default nextval('nexta_seq'),
> x varchar not null UNIQUE
> 
> );
> 
> create table b (
> 
>     z integer not null PRIMARY KEY default nextval('nextbz_seq'),
>     x varchar NOT NULL REFERENCES a(x),
> 
> );
> 
> 


-- 
#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#================================================== JanWieck@Yahoo.com #



Re: Unique Constraint with foreign Key

From
"Greg Patnude"
Date:
You've got it wrong when you reference the data column (a.x) -- your
foreign key should reference the primary key in the referenced table
(a.y)... 

Besides,  in your table A -- 1, 99 2, 99 violates your unique constraint
on column 'X' -- it would never happen... 

What I suggested is like this:

create table a (
y integer not null primary key default nextval('nexta_seq'),x varchar not null UNIQUE

);

create table b (
z integer not null PRIMARY KEY default nextval('nextbz_seq'),x integer NOT NULL REFERENCES a(y),
);

Table A would have 

1, 99
2, 99
..

99,99

and table B would have

1, 1
1, 2
..
1, 99








Greg Patnude / The Digital Demention
2916 East Upper Hayden Lake Road
Hayden Lake, ID. 83835
(208) 762-0762
Send replies to: gpatnude@adelphia.net

Website: http://www.left-center.com

-----Original Message-----
From: Jan Wieck [mailto:JanWieck@Yahoo.com] 
Sent: Tuesday, February 17, 2004 6:42 AM
To: Greg Patnude
Cc: pgsql-sql@postgresql.org
Subject: Re: [SQL] Unique Constraint with foreign Key

Greg Patnude wrote:
> Pleas also note that the referenced column in the foreign table either
needs
> to be the PRIMARY KEY or have a unique constraint on it or maybe it
just
> requires an index on it -- I'm not sure but I discovered that if the
column
> in the foreign table (containing the REFERENCED key...) is NOT the
primary
> key column -- the REFERENCES a(x) will faill unless a.x is specified
as
> 'UNIQUE' -- as in the following example:

This is according to the SQL specification, which doesn't like doubts.

Imagine rows (1, 99), (2, 99) in table a and row (3, 99) in table b. 
Which of the a-rows is now referenced and am I allowed to delete the 
other? There are good arguments either way, but if you require a UNIQUE 
on a.x, then this question will never come up.


Jan

> 
> create table a (
> y integer not null primary key default nextval('nexta_seq'),
> x varchar not null UNIQUE
> 
> );
> 
> create table b (
> 
>     z integer not null PRIMARY KEY default nextval('nextbz_seq'),
>     x varchar NOT NULL REFERENCES a(x),
> 
> );
> 
> 


-- 
#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#================================================== JanWieck@Yahoo.com #



Re: Unique Constraint with foreign Key

From
Karsten Hilbert
Date:
> > or Can i have varchar types  of size 50 as primary keys in Postgres.
> Yes. But unless the 50 character limit comes from a business rule, you
> might want to use the type 'text'.
And if that limit does come from a business rule you might want
to think twice whether using columns constrained by business
rules are good candidates for primary keys.

Karsten
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