Perl MySQL DBI-更新并获取上一个值

Question

I am using the Perl DBI module with MySQL and trying to get the initial value before adding 1 to it when updating a row.

If the current value was 1000 I need to return the value of 1000 and then add 1 to the value.

I use this statement in perl to use one transaction...

update TABLE_NAME set ID = (\@cur_value := ID) + 1

I know I can do a select then an update as two statements or lock the tables manually but transactions happen so fast on our platform that it may cause inconsistencies and this is the fastest way to do it.

However I simply cannot find a way to return the original value before the increment using this statement.

It works fine in ASP as below:

qry = "update V15_TRACKING set TRACKING_ID = (@cur_value := TRACKING_ID) + 1 where TRACKING_TYPE='ABC'"
Set oRS = oConn.Execute(qry)
qry = "select @cur_value"
if not oRS.EOF then
        while not oRS.EOF
            CurrTrackingID = oRs.Fields("@cur_value")
            oRS.movenext
        wend
    oRS.close
    end if

Please can someone advise me what I need to do to return the original value in Perl as I have searched everywhere and tried all sorts of solutions.

Answer 1

A snippet to show what you're actually doing in perl, and your result would help diagnose what is going on in your script.

I tried this trivial example:

The DB:

CREATE DATABASE TEST;

CREATE TABLE foo (
    id  int(11) NOT NULL AUTO_INCREMENT PRIMARY KEY,
    val int(11) NOT NULL
);

INSERT INTO foo (val) VALUES (1);

And the Perl

#!/bin/env perl
use strict;
use warnings;

use Data::Dumper;
use DBI;

my $dbh = DBI->connect('DBI:mysql:database=test', 'dbuser', 'dbpass');

my $select = $dbh->prepare('SELECT * FROM foo WHERE id=?');
my $select_old_val = $dbh->prepare('SELECT @old_val');
my $update = $dbh->prepare('UPDATE foo SET val=(@old_val := val) + 1 WHERE id=?');

$update->execute(1);
$select_old_val->execute();
$select->execute(1);

while (my $row = $select_old_val->fetchrow_hashref) {
    print Dumper $row;
}


while (my $row = $select->fetchrow_hashref) {
    print Dumper $row;
}

And after a few goes:

$ perl select_and_update.pl 
$VAR1 = {
          '@old_val' => '10'
        };
$VAR1 = {
          'id' => '1',
          'val' => '11'
        };