Sql 哪种查询更有效？内部连接与子查询？求和与拥有

我正在使用PostgreSql 9。我有一个简单的问题。 哪种查询更有效

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

您的第二个查询在语法上无效。不能从HAVING子句中的SELECT list输出列引用列别名

无论如何，这两个查询都不好。如果确实要查找不存在的用户id和操作的组合，请尝试：

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;

我在子查询t中引入了一个具有单行的派生表，并将其左连接到两个基表的组合中。如果组合不存在，则仅返回一行。 假设列名称user_id和action在两个表中只出现一次。否则，请对第二个联接使用更明确的条件：

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

详情：

可供替代的 可能更快，但：

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;

---

在本例中，无论采用哪种方式，使用构造都是安全的。

另一个解决方案使用的是NOT EXISTS，这是您所问问题的精确翻译：

请提供不存在操作2的会话：

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 
 ) 

---

你觉得这个怎么样

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 

我的一便士价值

我可以用两种不同的方式来解释你的意图，并且对每种情况都有不同的解决方案，我的首选解决方案在每种情况下都是第一位的

至少有一个历史记录的会话，其中操作2 不管有多少历史记录，行动=2

或

---

检查explain analyze的输出，verboseWell，你试过了吗？第二条似乎都不正确。测试他们，写下所有的结果，然后问一个好问题。你对博士后的看法？9不是有效的版本。“9.2或9.3是……我认为第二个查询是不好的。”KamilJ——纯粹是对欧文观点的评论；正确的语法应该是让SUM1=0重复HAVING子句中SELECT子句的逻辑，或者如果您不想重复自己的话；选择*作为聚合，其中s=0。你真的喜欢使用？当您的查询还包括范围查找等时，它不是真的变得不整洁吗？@MatBailie:我对简短、简洁的代码有一个偏爱。对于持久化查询，更安全的做法是使用显式表单，以减少以后对基础表的架构更改的影响。不过，示例中的第一个USING子句是安全的。会话\u id是否可能在会话\u历史记录中没有对应的行？如果是这样，根据这里的其他答案，您需要左外部联接或不存在类型的查找。这里有很多要点。会话\u id在会话\u历史记录中是否可能有多个对应的行？如果是这样，SH2子查询应该有一个独特的or GROUP BY，以防止结果重复。

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 

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

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
               )

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

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

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
                   )