in reply to Re: Re: Querying Meta Data
in thread Querying Meta Data

Thanks dragonchild, i did mean to mention that i had the sql for the first link and not the others.

and Thanks simonm thats exactly what I had in mind but I had no clue that you could join a table more than once. Off i go looking thre MySQL documentation with a new point of view. How many joins do you do in your case? How many is too many? Just ideas, I know that it will vary from case to case, and depend on the size of the table etc.

Here is the code i came up with so far. Its a bit rough, but i thought others might find it useful!

use Data::Dumper;
print GetNodes({name => "hello",type => "god", color => "greeen"});

sub GetNodes
{
    my $meta = shift;   #expects a hash of {name => value}
    print Dumper($meta);
    my $sql = "SELECT md1.node_id FROM ";
    my @From;           # list of tables
    my @Where;          # list of conditions
    my @LastWhere;
    my $i = 0;
    foreach my $name (keys %$meta)
    {
        $i ++;
        push @From, "`meta_data` as md$i";
        push @Where,"md$i.name = \"" . $name . "\"";
        push @Where,"md$i.value = \"" . $meta->{$name} . "\"";
        push @LastWhere,"md$i.node_id";
    }
    $sql .= join(",",@From) . " WHERE ";
    $sql .= join(" AND ", @Where);
    $sql .= " AND " .join(" = ",@LastWhere) . ";";
    return $sql;
}

[download]
outputs 
SELECT md1.node_id
 FROM `meta_data` as md1,`meta_data` as md2,`meta_data` as md3
 WHERE md1.name = "type"
 AND md1.value = "god"
 AND md2.name = "color"
 AND md2.value = "greeen"
 AND md3.name = "name"
 AND md3.value = "hello"
 AND md1.node_id = md2.node_id = md3.node_id;

[download]
Thanks!
Update: Added above code :-)
Eric Hodges

Replies are listed 'Best First'.
Re: Re: Re: Re: Querying Meta Data
by simonm (Vicar) on Jul 24, 2003 at 23:39 UTC

    How many joins do you do in your case?

    Like your example code, I use as many joins as I have distinct criteria. We typically only have a couple of sets of criteria, but sometimes it's five or six, and the performance on MySQL has been fine.

    Here is the code i came up with so far.

    I'd like to reiterate the importance of using placeholders, which allow for efficient query plan caching.

    You could easily modify your sample code to return a parameterized query based simply on how many criteria pairs you had. You can then pass your pairs of meta-data criteria to the DBI as execute() parameters. It eliminates any worries about quoting and escaping the values, and may provide a performance benefit by simplifying the database's query parse/plan efforts. 

    sub GetNodes {
  my $meta = shift;
  my $sql = buildMetaQuery( scalar keys %$meta );
  my $sth = $dbh->prepare_cached($sql);
  $sth->execute( %$meta );
  $sth->fetchrow_array  
}

    [download]

    This assumes that buildMetaQuery( 3 ) returns something like: 

    SELECT md1.node_id
 FROM `meta_data` as md1,`meta_data` as md2,`meta_data` as md3
 WHERE md1.name = ?
   AND md1.value = ?
   AND md2.name = ?
   AND md2.value = ?
   AND md3.name = ?
   AND md3.value = ?
   AND md1.node_id = md2.node_id = md3.node_id;

    [download]
[reply]
[d/l]
[select]

      Thanks I was trying to come up with a way to create the query without calling it. Like your buildMetaQuery, but I didn't remember I could use a hash in a list context like that. I will certainly revamp my code to that model as i completly understand the need for placeholders. That was just my baby steps approach, produce the rigth query first, then figure out how to produce it better. Thanks agian!

      ___________
      Eric Hodges
[reply]

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