$query1 = 'select Name from Customers where CustId = :custid';
$query2 = 'select Name from Sales where SalesId = :salesid';

$sth1 = $dbh->prepare($query1);
$sth2 = $dbh->prepare($query2);

if ($x > 10) {
  $sth1->bind_param('custid', $custid);
  $sth1->execute();
  while ($row = $sth1->fetchrow_hashref) {
    ...
  }
} else {
  $sth2->bind_param('salesid', $salesid);
  $sth2->execute();
  while ($row = $sth2->fetchrow_hashref) {
    ...
  }
}
[download]

Like "prepare" except that the statement handle returned will be store
+d in a hash associated with the $dbh. If another call is made to "pre
+pare_cached" with the same $statement and %attr parameter values, the
+n the corresponding cached $sth will be returned without contacting t
+he database server.
[download]

if ($x>10) { $query="(select Name from Customers where CustId='$custid
+')" } else { $query="(select Name from Sales where SalesId='$salesid'
+ and CustId='$custid')" }
[download]

$sth->execute(?)
[download]

$sth->execute(?,?)
[download]

Did you notice that my version has two separate statement handles that are being executed? You can do the same thing with yours. Prepared statements with placeholders will likely be faster than constructing queries on the fly and executing them, and certainly much safer.

How many possible queries can you need to construct that you can't afford to use prepared statements and placeholders?

prepared statements with placeholders will likely be faster than constructing queries on the fly

I don't think rather doubt this is generally true. It would be nice to see it demonstrated.

my ( $query, @args );
if ( $x > 10 ) {
    $query = 'select Name from Customers where CustID = ?';
    @args = ( $custid );
} else {
    $query = 'select Name from Sales where SalesId = ? and CustID = ?'
+;
    @args = ( $salesid, $custid );
}

my $sth = $dbh->prepare( $query );
$sth->execute( @args );
[download]