将MySQL查询从UNION转换为更高效的东西

Question

我在我的网站上有一个自定义消息传递功能，并且当显示用户拥有的消息数时，我使用联合查询来这样做，因为有两种类型的消息。

我这样做如下：

SELECT COUNT(m.messageid) type_a_messagecount 
FROM messages m, 
messagerecipients r 
WHERE m.messageid = r.messageid AND 
(
m.type = 0 OR 
m.type = 2 
) AND 
r.read = 0 AND 
r.trashed = 0 AND 
r.deleted = 0 AND 
r.userid = 1 AND 
r.authorid <> 1 
UNION 
SELECT COUNT(m.messageid) type_b_messagecount 
FROM messages m, 
messagerecipients r 
WHERE m.messageid = r.messageid AND 
(
m.type = 1 
) AND 
r.read = 0 AND 
r.trashed = 0 AND 
r.deleted = 0 AND 
r.userid = 1 AND 
r.authorid <> 1 

这工作，但东西告诉我它的效率极其低下。

有没有一种方法来简化这个或我坚持使用UNION查询？

谢谢。

Answer 1

Select Sum(Case When M.type In(0,2) Then 1 Else 0 End) As type_a_messagecount 
    , Sum(Case When M.type = 1 Then 1 Else 0 End) As type_b_messagecount 
From messages As M 
    Join messagerecipients As R 
     On R.messageid = M.messageid 
Where R.read = 0 
    And R.trashed = 0 
    And R.deleted = 0 
    And R.userid = 1 
    And R.authorid <> 1 

Answer 2

什么

SELECT COUNT(m.messageid),m.type 
FROM messages m, 
    messagerecipients r 
WHERE m.messageid = r.messageid AND 
    m.type IN (0,1,2) AND 
    r.read = 0 AND 
    r.trashed = 0 AND 
    r.deleted = 0 AND 
    r.userid = 1 AND 
    r.authorid <> 1 
GROUP BY m.type 

然后以编程方式增加对0型和2

Answer 3

子查询的结果与一个case语句会给你想要的东西：

SELECT 
    count(m.messageid), type_ 
FROM 
    (
     SELECT 
      m.messageid 
      CASE 
       WHEN m.type = 0 THEN 'a' 
       WHEN m.type = 1 THEN 'b' 
       WHEN m.type = 2 THEN 'a' 
      END as type_ 
      FROM 
       messages m,messagerecipients r 
      WHERE 
       m.messageid = r.messageid 
       AND r.read = 0 
       AND r.trashed = 0 
       AND r.deleted = 0 
       AND r.userid = 1 AND 
       AND r.authorid <> 1 
    ) as t 
GROUP BY type_