Question

我有以下（相当遗留，一年或两年前做过）SQL查询。 SQL所在的网页（在ASP.NET中的SqlDataSource / GridView上使用）非常慢，我已经确定了这个查询的缓慢 - 看似是因为子选择。我已经尝试过使用连接来加快速度，但我无法让它工作。有任何想法吗？

我不会把整个查询放在这里，只是因为我在一台没有互联网访问权限的机器上工作，所以我无法复制和粘贴，而且大多数只是从主表中选择。

SELECT ..., 
       CASE 
         WHEN di.Total = di.Delivered THEN 'Received' 
         ELSE 'Not Received' END AS 'Status',
       ...
FROM Deliveries AS d 
  LEFT OUTER JOIN (
         SELECT Delivery, 
                COUNT(*) AS Total, 
                COUNT(CASE WHEN Status = 2 THEN 1 END) AS Delivered 
         FROM DeliveryItems 
         GROUP BY Delivery
  ) AS di ON d.ID = di.Delivery

任何提示？

Answer 1

SELECT Delivery, 
       COUNT(*) AS Total, 
       COUNT(CASE WHEN Status = 2 THEN 1 END) AS Delivered 
FROM DeliveryItems 
GROUP BY Delivery

这部分可以通过抛弃CASE声明来改进。也许尝试连接两次，一次针对总计数，一次针对过滤计数。

SELECT ..., 
       CASE 
         WHEN d2.Total = d1.Delivered THEN 'Received' 
         ELSE 'Not Received' END AS 'Status',
       ...
FROM Deliveries AS d 

LEFT OUTER JOIN (

SELECT Delivery,
       COUNT(*) AS Delivered
FROM DeliveryItems
WHERE Status = 2
GROUP BY Delivery ) d1 ON d.Id = d1.Delivery

LEFT OUTER JOIN (

SELECT Delivery,
       COUNT(*) AS Total
FROM DeliveryItems
GROUP BY Delivery ) d2 ON d.ID = d2.Delivery

加速这个SQL查询

1 个答案: