Обсуждение: INSERT only under certain conditions (SELECT)

Hi,

I want to insert a row only under condition that there isn't already
another row with similar values -- something like a INSERT INTO
... WHERE NOT EXISTS (SELECT ...)?

Hoping for help,
joachim

create an uniq index on the columns,

# create unique index index_name on table (col1,col2,col3,...)


hope it helps.


regds
mallah

On Monday 08 July 2002 06:48, Joachim Trinkwitz wrote:
> Hi,
>
> I want to insert a row only under condition that there isn't already
> another row with similar values -- something like a INSERT INTO
> ... WHERE NOT EXISTS (SELECT ...)?
>
> Hoping for help,
> joachim
>
>
>
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On Mon, Jul 08, 2002 at 03:18:33 +0200, Joachim Trinkwitz <jtr@uni-bonn.de> wrote:
> Hi,
> 
> I want to insert a row only under condition that there isn't already
> another row with similar values -- something like a INSERT INTO
> ... WHERE NOT EXISTS (SELECT ...)?

You can do something like the following:
insert into tab (col1, col2, col3) select 'val1', 'val2', 'val3' where
not exists (select * from tab where col1 = 'val1' and col2 = 'val2' and
col3 = 'val3');

Maybe I should explain my problem a little bit: simplified, I have two
tables, L and T, (which values references to L):

L:
art|kvvnr|semester---+-----+--------7.0| 4151| 2002ws7.0| 4326| 2002ws6.1| 4200| 2002ws7.0| 4151| 2001ws

(combination of kvvnr and semester is unique)

T:
 id |semester|kvvnr----+--------+----- 123| 2002ws |4151 123| 2001ws |4151

Now I want to allow a user with id 123 only to insert a row in T if he
doesn't choose values from L with same 'art' and 'semester' values --
in the examples user 123 has already chosen a kvvnr from semester
2002ws in category 'art', so he shouldn't be allowed to insert the
values of the second row in table L.

I hope, someone has followed this complicated thing up to here and
there is a solution for my problem.

Greetings and thanks for your answers by now and further on,
joachim

On Mon, Jul 08, 2002 at 17:49:56 +0200, Joachim Trinkwitz <jtr@uni-bonn.de> wrote:
> 
> Now I want to allow a user with id 123 only to insert a row in T if he
> doesn't choose values from L with same 'art' and 'semester' values --
> in the examples user 123 has already chosen a kvvnr from semester
> 2002ws in category 'art', so he shouldn't be allowed to insert the
> values of the second row in table L.

In the database you enforce this using a unique index.
The application needs to be able to handle failures when you try to
insert a duplicate key.

If you can't afford to have a transaction fail part way through, then
anything that writes that table should do a table lock at the start
of the transaction.