使用从另一个表中检索的数据从表中检索数据

Question

I need to get the id from one table in a database and use that id as a "key" in order to get the name from another table. For example

 $result2 = mysql_query("SELECT * FROM XXX WHERE id=8 ") 
 or die(mysql_error());  

 while($row = mysql_fetch_array( $result2 )) {

echo $row['client_id'];

Here i get the client_id from the table 1. Now how do i implement that into this so that it returns the name and last name only of those that have the above id

$data = mysql_query("SELECT firstname, lastname FROM YYY WHERE ???? :S ") 
or die(mysql_error()); 
Print "<table border cellpadding=3>"; 
while($info = mysql_fetch_array( $data )) 
   { 
   Print "<tr>"; 
   Print "<th>fname:</th> <td>".$info['firstname'] . "</td> "; 
   Print "<th>lname:</th> <td>".$info['lastname'] . " </td></tr>"; 
} 
Print "</table>"; 
?>

Answer 1

If you only need to take the client_id from XXX table, use a subquery:

$data = mysql_query("SELECT firstname, lastname FROM YYY WHERE id IN (SELECT client_id FROM XXX WHERE id=8 )") .

If XXX.id is PRIMARY KEY, then that subquery will return just one row, so instead if using id IN (SEL...), use id = (SEL...