I need help optimizing some sql queries.
I have a site that is query extensive and I need to quicken it up.
The problem lies in the fact that Im querying a database with about 20,000 rows,
Then I have to check each entry against another table.

Lets say this is the main query, Its going through the table 'leads'
$x = mysql_query("select `ID` FROM `leads` ");

Im using a 'while' statement to fetch_arrays from that table.
While ($z = $mysql_fetch_array($x)){
Echo $z['ID'];
}

pretty simple right.
Well now I have to check if '$x['ID']' is in another database That has also 20,000+ entries.

This is an idea of what it looks like.

$x = mysql_query("select `ID` FROM `leads` ");
While ($z = $mysql_fetch_array($x)){

---> $zz = mysql_query("SELECT count(*) FROM `table_2' Where (loanid = '".$z['ID']."' )");
---> $count = mysql_result($cart,0);
echo $count;
}

this is just a sample and not the real code.

basiclly its querying over 20,000 times while looping through.
I need to combine the tables or something.

use a left outer join so that it will return a 0 when there are no matching loans for a given ID