MySQL查询从子表中获取最新值

Question

我有一张客户表和三张相关详细信息表，如Fiddle所示。

我想要做的是获取客户的数据及其相关行最新联系日期。

我想从该查询创建一个视图，并能够按操作员ID进行过滤。我总是得到错误的行数到我所期望的。

以下是我的查询，有人可以帮我找出为什么它返回不正确的结果？

select c.*,cs.client_status,cn.client_note,cd.contact_date from clienti c 
left join client_status cs on c.id = cs.client 
left join client_notes cn on c.id = cn.client 
left join client_contact_date cd on c.id = cd.client 
where 
(
    (
     `cd`.`contact_date` = (select max(`client_contact_date`.`contact_date`) from `client_contact_date` where `client_contact_date`.`client` = `c`.`id`) 
    ) 
    AND 
    (
     `cn`.`mod_time` = (select max(`client_notes`.`mod_time`) from `client_notes` where `client_notes`.`client` = `c`.`id`) 
    ) 
    AND 
    (
     `cs`.`mod_time` = (select max(`client_status`.`mod_time`) from `client_status` where `client_status`.`client` = `c`.`id`) 
    ) 
) 

Answer 1

我建议你把它分成小块，然后开始把连接放在一起。

所有你想要的内容都是每个客户端的最新contact_date/status/note。你可以得到每个那些使用聚合：

SELECT client, MAX(contact_date) AS latestContact 
FROM client_contact_date 
GROUP BY client; 

SELECT client, MAX(mod_time) AS latestNote 
FROM client_notes 
GROUP BY client; 

SELECT client, MAX(mod_time) AS latestStatus 
FROM client_status 
GROUP BY client; 

第一个查询很容易被外部联接到客户表以获得客户信息，因为有只应为每个客户一行。在过去两年的子查询，你就必须加入回到原来的表（笔记，状态）来获取这样的信息，因为该行必须用时间，以及匹配：一旦

SELECT n.client, n.client_note, n.mod_time 
FROM client_notes n 
JOIN(
    SELECT client, MAX(mod_time) AS latestNote 
    FROM client_notes 
    GROUP BY client) t ON t.client = n.client AND t.latestNote = n.mod_time; 

SELECT s.client, s.client_status, s.mod_time 
FROM client_status s 
JOIN(
    SELECT client, MAX(mod_time) AS latestStatus 
    FROM client_status 
    GROUP BY client) t ON t.client = s.client AND t.latestStatus = s.mod_time; 

你把所有的这些东西，你可以使用外部联接带来最新的信息一起行：

SELECT c.id, c.clientName, c.operators, cd.latestContact, cn.client_note, cn.latestNote, cs.client_status, cs.latestStatus 
FROM clienti c 
LEFT JOIN(
    SELECT client, MAX(contact_date) AS latestContact 
    FROM client_contact_date 
    GROUP BY client) cd ON cd.client = c.id 
LEFT JOIN(
    SELECT n.client, n.client_note, n.mod_time AS latestNote 
    FROM client_notes n 
    JOIN(
    SELECT client, MAX(mod_time) AS latestNote 
    FROM client_notes 
    GROUP BY client) t ON t.client = n.client AND t.latestNote = n.mod_time) cn ON cn.client = c.id 
LEFT JOIN(
    SELECT s.client, s.client_status, s.mod_time AS latestStatus 
    FROM client_status s 
    JOIN(
    SELECT client, MAX(mod_time) AS latestStatus 
    FROM client_status 
    GROUP BY client) t ON t.client = s.client AND t.latestStatus = s.mod_time) cs ON cs.client = c.id; 

我想提出一个意见，但。我看到您将运营商存储为列表。这打破了正常化，通常是一个非常糟糕的主意。有关我的信息，请参阅Is storing a delimited list in a database column really that bad?

以下是我的查询的示例SQL Fiddle。