我对查询优化了解不多，但我知道执行查询的顺序
这是我写的查询
SELECT
`main_table`.forum_id,
my_topics.topic_id,
(
SELECT MAX(my_posts.post_id) FROM my_posts WHERE my_topics.topic_id = my_posts.topic_id
) AS `maxpostid`,
(
SELECT my_posts.admin_user_id FROM my_posts WHERE my_topics.topic_id = my_posts.topic_id ORDER BY my_posts.post_id DESC LIMIT 1
) AS `admin_user_id`,
(
SELECT my_posts.user_id FROM my_posts WHERE my_topics.topic_id = my_posts.topic_id ORDER BY my_posts.post_id DESC LIMIT 1
) AS `user_id`,
(
SELECT COUNT(my_topics.topic_id) FROM my_topics WHERE my_topics.forum_id = main_table.forum_id ORDER BY my_topics.forum_id DESC LIMIT 1
) AS `topicscount`,
(
SELECT COUNT(my_posts.post_id) FROM my_posts WHERE my_topics.topic_id = my_posts.topic_id ORDER BY my_topics.topic_id DESC LIMIT 1
) AS `postcount`,
(
SELECT CONCAT(admin_user.firstname,' ',admin_user.lastname) FROM admin_user INNER JOIN my_posts ON my_posts.admin_user_id = admin_user.user_id WHERE my_posts.post_id = maxpostid ORDER BY my_posts.post_id DESC LIMIT 1
) AS `adminname`,
(
SELECT forum_user.nick_name FROM forum_user INNER JOIN my_posts ON my_posts.user_id = forum_user.user_id WHERE my_posts.post_id = maxpostid ORDER BY my_posts.post_id DESC LIMIT 1
) AS `nickname`,
(
SELECT CONCAT(ce1.value,' ',ce2.value) AS fullname FROM my_posts INNER JOIN customer_entity_varchar AS ce1 ON ce1.entity_id = my_posts.user_id INNER JOIN customer_entity_varchar AS ce2 ON ce2.entity_id=my_posts.user_id WHERE (ce1.attribute_id = 1) AND (ce2.attribute_id = 2) AND my_posts.post_id = maxpostid ORDER BY my_posts.post_id DESC LIMIT 1
) AS `fullname`
FROM `my_forums` AS `main_table`
LEFT JOIN `my_topics` ON main_table.forum_id = my_topics.forum_id
WHERE (forum_status = '1')
现在我想知道是否有任何方法可以优化它？因为所有逻辑都是用Select部分而不是From编写的，但我不知道如何在查询的From部分编写相同的逻辑？
是否有任何区别或两者相同？
由于
答案 0 :(得分：2)
相关的子查询应该是最后的手段，它们通常最终会被执行RBAR，并且考虑到许多子查询非常相似，尝试使用连接获得相同的结果将导致更少的表扫描
我注意到的第一件事是你的所有子查询都包含表my_posts，大多数包含ORDER BY my_posts.post_id DESC LIMIT 1，那些没有计数但没有组的那些，所以顺序和限制是无论如何多余，所以我的第一步是加入my_posts：
SELECT *
FROM my_forums AS f
LEFT JOIN my_topics AS t
ON f.forum_id = t.forum_id
LEFT JOIN
( SELECT topic_id, MAX(post_id) AS post_id
FROM my_posts
GROUP BY topic_id
) AS Maxp
ON Maxp.topic_id = t.topic_id
LEFT JOIN my_posts AS p
ON p.post_id = Maxp.post_id
WHERE forum_status = '1';
此子查询只是确保您获得每个topic_id的最新帖子。为方便起见，我在这里缩短了表别名，我不确定为什么要使用比实际表名更长的表别名？
现在你有大量的查询可以开始在你的列中添加，为了获得帖子计数，我已经为子查询添加了一个计数Maxp，我还需要添加更多加入以获取一些细节，例如名称：
SELECT f.forum_id,
t.topic_id,
p.post_id AS `maxpostid`,
p.admin_user_id,
p.user_id,
t2.topicscount,
maxp.postcount,
CONCAT(au.firstname,' ',au.lastname) AS adminname,
fu.nick_name AS nickname
CONCAT(ce1.value,' ',ce2.value) AS fullname
FROM my_forums AS f
LEFT JOIN my_topics AS t
ON f.forum_id = t.forum_id
LEFT JOIN
( SELECT topic_id,
MAX(post_id) AS post_id,
COUNT(*) AS postcount
FROM my_posts
GROUP BY topic_id
) AS Maxp
ON Maxp.topic_id = t.topic_id
LEFT JOIN my_posts AS p
ON p.post_id = Maxp.post_id
LEFT JOIN admin_user AS au
ON au.admin_user_id = p.admin_user_id
LEFT JOIN forum_user AS fu
ON fu.user_id = p.user_id
LEFT JOIN customer_entity_varchar AS ce1
ON ce1.entity_id = p.user_id
AND ce1.attribute_id = 1
LEFT JOIN customer_entity_varchar AS ce2
ON ce2.entity_id = p.user_id
AND ce2.attribute_id = 2
LEFT JOIN
( SELECT forum_id, COUNT(*) AS topicscount
FROM my_topics
GROUP BY forum_id
) AS t2
ON t2.forum_id = f.forum_id
WHERE forum_status = '1';
我不熟悉您的架构，所以上面可能需要一些调整，但主要仍然是 - 使用JOIN而不是子选择。
我要做的下一个优化阶段是摆脱你的customer_entity_varchar表，或至少停止使用它来存储基本的名字和姓氏。 Entity-Attribute-Value模型是一个SQL反模式，如果您向FirstName表添加了两列LastName和forum_user，您将立即从查询中丢失两个连接。我不会过多地参与EAV vs Relational debate，因为这已被多次广泛讨论过，我没有其他任何补充。
最后一个阶段是添加适当的索引，你最好决定什么是合适的，我建议你可能至少需要索引每个表中的外键，可能更多。
修改
要获得每forum_id行一行，您需要使用以下内容：
SELECT f.forum_id,
t.topic_id,
p.post_id AS `maxpostid`,
p.admin_user_id,
p.user_id,
MaxT.topicscount,
maxp.postcount,
CONCAT(au.firstname,' ',au.lastname) AS adminname,
fu.nick_name AS nickname
CONCAT(ce1.value,' ',ce2.value) AS fullname
FROM my_forums AS f
LEFT JOIN
( SELECT t.forum_id,
COUNT(DISTINCT t.topic_id) AS topicscount,
COUNT(*) AS postCount,
MAX(t.topic_ID) AS topic_id
FROM my_topics AS t
INNER JOIN my_posts AS p
ON p.topic_id = p.topic_id
GROUP BY t.forum_id
) AS MaxT
ON MaxT.forum_id = f.forum_id
LEFT JOIN my_topics AS t
ON t.topic_ID = Maxt.topic_ID
LEFT JOIN
( SELECT topic_id, MAX(post_id) AS post_id
FROM my_posts
GROUP BY topic_id
) AS Maxp
ON Maxp.topic_id = t.topic_id
LEFT JOIN my_posts AS p
ON p.post_id = Maxp.post_id
LEFT JOIN admin_user AS au
ON au.admin_user_id = p.admin_user_id
LEFT JOIN forum_user AS fu
ON fu.user_id = p.user_id
LEFT JOIN customer_entity_varchar AS ce1
ON ce1.entity_id = p.user_id
AND ce1.attribute_id = 1
LEFT JOIN customer_entity_varchar AS ce2
ON ce2.entity_id = p.user_id
AND ce2.attribute_id = 2
WHERE forum_status = '1';