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Re: Conditionally executing multiple statements in series as single SQL statement

From: Sean Davis <sdavis2(at)mail(dot)nih(dot)gov>
To: Nathaniel Trellice <naptrel(at)yahoo(dot)co(dot)uk>
Cc: "pgsql-novice(at)postgresql(dot)org" <pgsql-novice(at)postgresql(dot)org>
Subject: Re: Conditionally executing multiple statements in series as single SQL statement
Date: 2009-12-18 13:16:33
Message-ID: 264855a00912180516y699b8025x2dec220c29f24a55@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-novice
On Fri, Dec 18, 2009 at 8:01 AM, Nathaniel Trellice <naptrel(at)yahoo(dot)co(dot)uk> wrote:
> Hi all,
>
> In C, and many other programming languages, statements like the following are popular:
>
> int status = (do_first_thing() && do_second_thing() && do_third_thing() && do_fourth_thing());
>
> With this kind of expression, the program calls the function 'do_first_thing'. If, and only if, that returns non-zero, 'do_second_thing' will be executed. Again, if and only if that returns non-zero, 'do_third_thing' is executed. Etc.
>
> In other words, later statements will only be executed if all before them have 'gone well'. When a statement 'fails', no further expressions are executed.. The variable 'status' is non-zero if, and only if, all four things were successfully executed.
>
> For convenience, I'd really like to be able to achieve similar behaviour within an SQL statement, i.e. present multiple statements (such as INSERT statements) and only execute the later ones if the earlier ones have been executed without error. And I'd like to be able to present all the statements within a single, compound SQL statement to the database.
>
> Is such a thing possible, using any fancy SQL syntactic tricks?

No tricks necessary.  What you are describing is called a transaction.

CREATE TABLE testing (
  id integer,
  name text unique
);

BEGIN;
INSERT INTO testing(id,name) values (1,'Bob');
INSERT INTO testing(id,name) values (2,'Joe');
INSERT INTO testing(id,name) values (3,'Sally');
COMMIT;

BEGIN;
INSERT INTO testing(id,name) values (4,'Ann');
-- the next statement will cause an error
-- due to violation of the unique constraint
INSERT INTO testing(id,name) values (5,'Bob');
-- We do a rollback, which will put the database
-- back into the state it was in just before the
-- second BEGIN statement
ROLLBACK;

SELECT * FROM TESTING;

See the documentation and Google about transactions.

Sean

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Next:From: Rory Campbell-LangeDate: 2009-12-18 13:18:45
Subject: Re: Conditionally executing multiple statements in series as single SQL statement
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Subject: Conditionally executing multiple statements in series as single SQL statement

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