x_mapping
xkey min_x  max_x
1 -9999 1
2 1 3.3

y_mapping
ykey min_y  max_y
1 -9999 2.5  1
2 2.5 3.5

##</code><code>##

  my $xkey = $dbh->selectrow_array("SELECT xkey FROM x_mapping WHERE min_x <= ? AND ? < max_x", {}, $x, $x );
  my $ykey = $dbh->selectrow_array("SELECT ykey FROM y_mapping WHERE min_y <= ? AND ? < max_y", {}, $y, $y );

##</code><code>##

x_mapping
xkey max_x
1 1.0
2 3.3

y_mapping
ykey max_y
1 2.5
2 3.5

  my $xkey = $dbh->selectrow_array("SELECT xkey FROM x_mapping WHERE ? < max_x ORDER BY max_x ASC LIMIT 1", {}, $x );
  my $ykey = $dbh->selectrow_array("SELECT ykey FROM y_mapping WHERE ? < max_y ORDER BY max_y ASC LIMIT 1", {}, $y );