MySQL左连接仅1行,具体取决于MAX()值 表1 表2:
我想要实现的是,我想要获得表1中的所有字段和表2中的左联接,但只获取最新的WorkDay值,如下所示:MySQL左连接仅1行,具体取决于MAX()值 表1 表2:,mysql,greatest-n-per-group,Mysql,Greatest N Per Group,我想要实现的是,我想要获得表1中的所有字段和表2中的左联接,但只获取最新的WorkDay值,如下所示: ID | Name | WorkDay | MissionCode 1 Brain NULL NULL 2 Amy NULL NULL 预期结果 到目前为止,我尝试的是: SELECT table1.*, t2.WorkDay, t2.MissionCode FROM table1 LEFT JOIN (S
ID | Name | WorkDay | MissionCode
1 Brain NULL NULL
2 Amy NULL NULL
预期结果
到目前为止,我尝试的是:
SELECT
table1.*, t2.WorkDay, t2.MissionCode
FROM
table1
LEFT JOIN
(SELECT
*
FROM
table2
ORDER BY
WorkDay DESC
LIMIT 0,1) AS t2
ON
t2.id = table1.id
但它从表2中返回空值,如下所示:
ID | Name | WorkDay | MissionCode
1 Brain NULL NULL
2 Amy NULL NULL
我测试了相同的查询,在内部联接的select命令中添加了额外的WHERE子句,结果成功了
SELECT
table1.*, t2.WorkDay, t2.MissionCode
FROM
table1
LEFT JOIN
(SELECT
*
FROM
table2
**WHERE id = 1**
ORDER BY
WorkDay DESC
LIMIT 0,1) AS t2
ON
t2.id = table1.id
对于第一行,它返回ok:
ID | Name | WorkDay | MissionCode
1 Brain 2019-02-01 2470
2 Amy NULL NULL
但是我不能用
WHERE id = table1.id
因为MySQL说
Unknown column 'table1.id' in 'where clause'
那么,正确的方法是什么呢?您可以使用一个简单的
JOIN
到table2
,只需将MAX(WorkDay)
条件作为一个相关的子查询放入JOIN
条件,在那里您可以访问table1
id值:
SELECT *
FROM table1 t1
JOIN table2 t2 ON t2.id = t1.id AND
t2.WorkDay = (SELECT MAX(WorkDay)
FROM table2
WHERE table2.id = t1.id)
输出:
ID Name ID WorkDay MissionCode
1 Brain 1 2019-02-01 2470
2 Amy 2 2019-02-01 7210
这也输出相同的结果,但请注意@Nick的答案更好
--1-------
select
t1.id,t1.name,t2.workday,t2.Missioncode
from t1
left join t2 on
t1.id=t2.id
and t2.workday =(select max(workday) from t2 );
select t1.id,t1.name,
(select max(workday) from t2 where t2.id=t1.id)as workday,
(select max(MissionCode) from t2 where t2.id=t1.id)as missioncode
from t1;
--2-------select
t1.id,t1.name,t2.workday,t2.Missioncode
from t1
left join t2 on
t1.id=t2.id
and t2.workday =(select max(workday) from t2 );
select t1.id,t1.name,
(select max(workday) from t2 where t2.id=t1.id)as workday,
(select max(MissionCode) from t2 where t2.id=t1.id)as missioncode
from t1;
“获取表2上的所有表1字段和左联接,但仅获取最新的工作日值”不清楚。使用足够的单词、句子和引用部分例子,清楚、完整地说出你的意思。当描述一个结果时:说足够多的话,有人可以离开并带着解决方案回来。请在代码问题中给出一个——剪切、粘贴和可运行的代码,加上所需的输出,再加上清晰的说明和解释。所以,给你展示的最小代码做你所期望的&在你出错的第一个地方给出最小的代码。(调试基础。)