Question

我正在使用PostgreSql 9.我有一个简单的问题。哪个查询更有效？

SELECT users_sessions.session_id, users_sessions.series 
FROM users_sessions 
WHERE users_sessions.user_id = 8 
AND users_sessions.session_id IN (
    SELECT session_id 
    FROM sessions_history 
    GROUP BY sessions_history.session_id 
    HAVING SUM(CASE WHEN sessions_history.action = 2 THEN 1 END) = 0
)

VS

SELECT US8.session_id,Us8.series

FROM

( SELECT us.session_id as S_ID, us.series 
        FROM users_sessions as US
        WHERE US.user_id = 8 ) AS US8
INNER JOIN
(SELECT SH.session_id as SH_ID
        FROM session_history as SH
        WHERE SH.action <> 2) AS SH2    
ON US8.session_id = SH2.session_id

Answer 1

您的第二个查询在语法上无效。您无法在SELECT子句中的HAVING列表（＆＃34;输出列＆＃34;）中引用列别名。

无论哪种方式，两个查询都不好。如果您确实想要找到不存在的(user_id, action)组合，请尝试改为：

SELECT t.*, 0 AS s
FROM  (SELECT 8 AS user_id, 2 AS action) t
LEFT  JOIN (
            users_sessions  us
       JOIN session_history sh USING (session_id)
           ) USING (user_id, action)
WHERE  sh.session_id IS NULL;

我将子查询t和LEFT JOIN中的单行的派生表引入到两个基表的组合中。如果组合不，则仅返回一行。假设列名user_id和action在两个表中只出现一次。否则，对第二个连接使用更明确的条件：

ON t.user_id = us.user_id AND t.action = sh.action

详细说明：

Select rows which are not present in other table

替代

可能更快，但是：

SELECT t.*, 0 AS s
FROM  (SELECT 8 AS user_id, 2 AS action) t
LEFT   JOIN users_sessions  us USING (user_id)
LEFT   JOIN session_history sh USING (action, session_id)
WHERE  sh.session_id IS NULL;

在此示例中，USING构造是安全的。

Answer 2

另一种解决方案使用NOT EXISTS，它是您似乎要求的精确翻译：

给我没有行动2的会议：

SELECT session_id, series 
FROM users_sessions as us
WHERE users_sessions.user_id = 8 
AND NOT EXISTS
 (  SELECT * 
    FROM sessions_history as sh
    WHERE action = 2
    AND us.session_id = sh.session_id 
 )

Answer 3

我的值得一提;

我可以用两种不同的方式解释你的意图，并为每个提供不同的解决方案（我的首选解决方案在每种情况下都是第一个）;

至少有一条历史记录的会话，其中的操作＆lt;＆gt; 2
（而不关心有多少历史记录，其中action = 2）

SELECT us.session_id, us.series FROM users_sessions AS us INNER JOIN session_history AS sh ON sh.session_id = us.session_id AND sh.action <> 2 WHERE us.user_id = 8 GROUP BY us.session_id, us.series

或

SELECT us.session_id, us.series FROM users_sessions AS us WHERE us.user_id = 8 AND EXISTS (SELECT * FROM session_history AS sh WHERE sh.session_id = us.session_id AND sh.action <> 2 )

或

SELECT us.session_id, us.series FROM users_sessions AS us INNER JOIN ( SELECT session_id FROM session_history WHERE sh.action <> 2 GROUP BY session_id ) AS sh ON sh.session_id = us.session_id WHERE us.user_id = 8

没有历史记录的会话记录where action = 2
（但可以有其他历史记录）

SELECT us.session_id, us.series FROM users_sessions AS us LEFT JOIN session_history AS sh ON sh.session_id = us.session_id AND sh.action = 2 WHERE us.user_id = 8 AND sh.session_id IS NULL -- No GROUP BY needed this time

或

SELECT us.session_id, us.series FROM users_sessions AS us WHERE us.user_id = 8 AND NOT EXISTS (SELECT * FROM session_history AS sh WHERE sh.session_id = us.session_id AND sh.action = 2 )

Answer 4

您如何看待这个？

SELECT US8.session_id,Us8.series

FROM

( SELECT us.session_id as S_ID, us.series 
        FROM users_sessions as US
        WHERE US.user_id = 8 ) AS US8
INNER JOIN
(SELECT SH.session_id as SH_ID
        FROM session_history as SH
        WHERE SH.action <> 2) AS SH2    
ON US8.session_id = SH2.session_id

哪个查询更有效？内部加入vs子查询？总和案例与

4 个答案:

替代