Php mysql-在一个查询中查询两个表，并创建两个结果的差异。

我目前正在PHP中运行两个查询，然后获取每个查询的结果以创建差异：

$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;

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是否可以轻松地将其合并到一个查询中？如何操作？

您需要单独运行每个查询，然后将它们作为派生表连接起来：

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;