Re: Mechanics of Update:Cascade

On Sat, 31 Jan 2004, beyaRecords - The home Urban music wrote:

> Hi,
> could someone please explain to me the mechanics of an UpDate:Cascade?
> Delete:Cascade I fully understand but not Update. I have 2 tables A and
> B. On B I have created a foreign key on user_id in both B and A for
> Update and Delete cascade. If I delete from A it deletes all from B. If
> I update A what happens in B?

Basically it means that if you update the referenced key in A, the
associated referencing values in B changes as well so as to attempt to
keep the associations the same.

Here's an example:

sszabo=# create table a(a int primary key, b int);
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index "a_pkey"
for table "a"
CREATE TABLE
sszabo=# create table b(a_key int references a on update cascade, b int);
CREATE TABLE
sszabo=# insert into a values (3, 4);
INSERT 535937 1
sszabo=# insert into a values (4, 5);
INSERT 535938 1
sszabo=# insert into b values (3,104);
INSERT 535939 1
sszabo=# insert into b values (4,105);
INSERT 535940 1
sszabo=# select * from a;
a | b
---+---
3 | 4
4 | 5
(2 rows)

sszabo=# select * from b;
a_key | b
-------+-----
3 | 104
4 | 105
(2 rows)

sszabo=# update a set a=a+100;
UPDATE 2
sszabo=# select * from a;
a | b
-----+---
103 | 4
104 | 5
(2 rows)

sszabo=# select * from b;
a_key | b
-------+-----
103 | 104
104 | 105
(2 rows)