mysql - querying 2 tables in one query and creating the difference of the two results.

Question

I'm currently running two queries in PHP, then take the result of each to create the difference:

$sec = 3600;    
$sql = "SELECT SUM(REVENUE) as C FROM REVENUE_LOG WHERE ENTRY_DATE BETWEEN (DATE_SUB(NOW(), INTERVAL $secs second)) AND (NOW())";
$res = $this->db->query($sql)->result_array();
$rev =  $res[0]['C'];

$sql = "SELECT SUM(COST) as C FROM COST_LOG WHERE ENTRY_DATE BETWEEN (DATE_SUB(NOW(), INTERVAL $secs second)) AND (NOW())";
$res = $this->db->query($sql)->result_array();
$cost =  $res[0]['C'];
$profit = $rev - $cost;

Is it possible to combine this into one query easily? How?

Answer 1

You need to run each query individually then JOIN them as derived tables:

SELECT R,C, R-C AS difference
FROM (
     SELECT SUM(REVENUE) as R
     FROM REVENUE_LOG 
     WHERE ENTRY_DATE BETWEEN (DATE_SUB(NOW(), INTERVAL $secs second)) AND (NOW())
     ) AS revenue
JOIN (
     SELECT SUM(COST) as C 
     FROM COST_LOG 
     WHERE ENTRY_DATE BETWEEN (DATE_SUB(NOW(), INTERVAL $secs second)) AND (NOW())
     ) AS cost;