Question

编写此查询的另一种方法是什么？

我的服务器需要花很多时间来执行这个：

SELECT c.id, c.franquicia_id, f.name, CONCAT(c.name,' ',c.surname) 
       contacto, c.created, DATE_FORMAT(c.created,'%d-%m-%Y') fecha 
FROM franquicias f, contacts c where f.id = c.franquicia_id 
 and c.created = (
                   select max(c2.created) 
                   from contacts c2 
                   WHERE c2.franquicia_id = c.franquicia_id
                 ) 
 and f.contract_id=2 
ORDER BY created DESC 
limit 10

Answer 1

此版本的子查询只需要为整个查询运行一次，而不是外部查询的每一行。

SELECT c.id, c.franquicia_id, f.name
   , CONCAT(c.name,' ',c.surname) AS contacto
   , c.created, DATE_FORMAT(c.created,'%d-%m-%Y') AS fecha 
FROM contacts AS c 
INNER JOIN franquicias AS f
   ON c.franquicia_id = f.id
    AND f.contract_id=2 
WHERE (c.franquicia_id, c.created) IN (
          SELECT franquicia_id, max(created) 
          FROM contacts
          GROUP BY franquicia_id
) 
ORDER BY created DESC 
LIMIT 10

或者，可以在子查询中使用子查询进行过滤。

f.contract_id=2可以放入ON或WHERE;但是（并且优化器可以消除任何差异），将其保留在ON可能可能减少在where中检查的行，并使以后更容易将查询转换为{{1如果需要找到＆＃34;最近没有f＆＃39; of contract_id 2＆＃34;。

注意：索引LEFT JOIN应该改进子查询，并使用(franquicia_id, created)来对付它;它还应该改善IN上的现有JOIN条件。

Answer 2

引用此 Post

我个人认为INNER JOIN更好，因为它更多可读。它表明了表格之间的关系。你得到了在连接中的那些关系，并在WHERE中进行过滤条款。这种分离使查询更具可读性。

SELECT 
    c.id, 
    c.franquicia_id, 
    f.name, 
    CONCAT(c.name,' ',c.surname) contacto, 
    c.created, 
    DATE_FORMAT(c.created,'%d-%m-%Y') fecha 
FROM franquicias f
INNER JOIN contacts c ON f.id = c.franquicia_id 
INNER JOIN 
(
    SELECT 
    C2.franquicia_id,
    MAX(C2.created) max_created
    FROM contacts C2 
    GROUP BY C2.franquicia_id
) AS maxCreatedForFrID
ON maxCreatedForFrID.franquicia_id = c.franquicia_id AND maxCreatedForFrID.max_created = c.created
WHERE f.contract_id = 2
ORDER BY created DESC
LIMIT 10;

注意：

franquicia_id,created表中的复合索引（contacts）可能会提高性能。

ALTER TABLE `contacts` ADD INDEX `idx_contacts_franquicia_id_created` ( `franquicia_id`, `created` );

Answer 3

如果franquicias包含的记录不多，则下一个查询必须更有效：

SELECT c.id, c.franquicia_id, f.name, CONCAT(c.name,' ',c.surname) 
       contacto, c.created, DATE_FORMAT(c.created,'%d-%m-%Y') fecha 
FROM (
    SELECT DISTINCT MAX(c2.created) as c2_created, f2.*
    FROM contacts c2
    JOIN franquicias f2 ON f2.id = c2.franquicia_id
) f
JOIN contacts c ON f.id = c.franquicia_id
AND c.created = f.c2_created
WHERE f.contract_id=2 
ORDER BY created DESC 
limit 10

编写此查询的另一种方法

3 个答案: