How to Perl Placeholder Variable for a DBD Mysql - perl

Question

I recently had some help with dbi and using placeholders in Perl for Mysql queries. However I am having an issue when using multiple statements for the previously declared or prepared variable in the dbi script.
Code:
use strict;
use warnings;
use DBI qw(:sql_types);
my $dbh = DBI->connect("DBI:mysql:...", ...);
## TABLE CREATION
$dbh->do("USE test;")
$dbh->do("CREATE TEMPORARY TABLE day5 (id INT, temp VARCHAR(4), time TIME, sumadd INT(11))");
$dbh->do("CREATE TEMPORARY TABLE humid (temp VARCHAR(4), i24 INT (10))");
$dbh->do("INSERT INTO day5 (id,temp,time) VALUES(1,'30','03:00:00') ");
$dbh->do("INSERT INTO humid (temp,i24) VALUES('30',8321) ");
## FAILING CODE
my $inter1 = 'i24'; # Generated value
my $sth = $dbh->prepare("SET \#sumadd5 = (SELECT ? FROM humid WHERE temp ='30') ");
$sth->bind_param( 1, $inter1 );
$sth->finish();
$dbh->do("UPDATE day5 SET sumadd= (SELECT \#sumadd5) WHERE time= '03:00:00' ");
my $sumadd = $dbh->selectrow_array("SELECT sumadd FROM day5");
print "$sumadd\n";
$dbh->disconnect();
$sumadd is undefined, but I expect 8321.
My question is how can I make it so the #sumadd5 variable can be inserted into the above query [UPDATE day5 SET sumadd=]? I have added some remarks in the syntax in the spirit of clarity.
IF I RUN MANUALLY BY APPLYING THE SYNTAX INTO MYSQL REMOVING PERL SPECIFICS MY TABLE UPDATE. IF I RUN THE SCRIPT NOTHING HAPPENS TO THE TABLE AND NO ERRORS ARE DISPLAYED.
I suspect that the break is with the UPDATE, however I can't confirm the $inter1 is being passed to the placeholder.

Answer 1

Problem #1
You never execute the statement you prepare. Use
my $sth = $dbh->prepare(q{...});
$sth->bind_param(1, $inter1);
$sth->execute(); <---
$sth->finish();
Shortcut #1:
my $sth = $dbh->prepare(q{...});
$sth->execute($inter1);
$sth->finish();
Shortcut #2:
$dbh->do(
q{...},
undef,
$inter1,
);
Problem #2
SELECT ? ...
behaves as if you had done
SELECT "i24" ... // As opposed to: SELECT i24 ...
just like
SELECT time ...
behaves as if you had done
SELECT '03:00:00' ...`
You can't build the SQL statement you are executing. You need to build it before you execute it. Use
$dbh->do(q{
SET #sumadd5 = (
SELECT }.( $dbh->quote_identifier($inter1) ).q{
FROM humid
WHERE temp = '30'
)
});