Re: Check before INSERT INTO

Shavonne Marietta Wijesinghe wrote:
> Even though n_gen is defined as a serial I can't let it handle the
> progressive key by its self since there is the need that some records
> should have the same value.

Of course you can - the default is only provided if you don't provide
your own.

> That's why i use 3 primary keys.

No, you're not. By definition you can only have one primary key. You are
using a 3-column primary key.

> A | B | C
> ---+---+---
> 1 | 1 | 1
> 2 | 1 | 3
> 2 | 2 | 3
> 2 | 3 | 3
> 3 | 1 | 2
> 3 | 2 | 2
> 4 | 1 | 1
>
> The 3 keys A, B, C are defined as Serial and Primay Keys

You don't have 3 keys, you have 3 columns and one primary key (A,B,C).

I'm not clear why B and C are serial - I don't see what that gets you.

> Anyway the other suggestion, Blocking the second user from reading the
> db. So for the second user I could give A temp key something like 0 and
> then do a select before the submit and change the value.
>
> But how is it possible to know if some other user is reading the db??

Don't try. If you want to insert some rows with a new value for A do
something like:

INSERT INTO my_shevi (a,b,c) VALUES (DEFAULT, 1, 1);
or
INSERT INTO my_shevi (a,b,c) VALUES (nextval(<sequence-name-here>, 1, 1);

INSERT INTO my_shevi (a,b,c) VALUES (currval(<sequence-name-here>), 1, 2);
INSERT INTO my_shevi (a,b,c) VALUES (currval(<sequence-name-here>), 1, 3);
etc.

Relevant parts of the manual (in 8.3 anyway):
9.15. Sequence Manipulation Functions
9.22. System Information Functions
The second chapter is for pg_get_serial_sequence() which might be useful
if you need to do this a lot.

Oh - and please try trimming unwanted parts of the message when you
reply. There was 100 unnecessary lines below here.

--
Richard Huxton
Archonet Ltd