MySQL LEFT JOIN只有1行，具体取决于MAX（）值

Question

表格1

ID | Name
1    Brain
2    Amy

表2：

ID | WorkDay    | MissionCode
1    2019-01-01    2360
1    2019-02-01    2470
2    2019-01-01    4470
2    2019-02-01    7210

我想要实现的是，我想得到所有表1字段并在表2上保持连接但只有最新的WorkDay值，如下所示：

预期结果

ID | Name | WorkDay    | MissionCode
1    Brain  2019-02-01   2470
2    Amy    2019-02-01   7210

到目前为止我尝试过的是：

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table2返回NULL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，并且它成功了。

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

它返回ok，对于第一行：

ID | Name | WorkDay    | MissionCode
1    Brain  2019-02-01   2470
2    Amy    NULL         NULL

但我不能用

WHERE id = table1.id 

因为MySQL说

Unknown column 'table1.id' in 'where clause'

那么，这是正确的方法是什么？

Answer 1

您可以使用简单的JOIN来table2，只需将MAX(WorkDay)条件作为相关子查询放入JOIN条件，您可以在其中访问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

Demo on dbfiddle

Answer 2

这也输出相同但注意@ 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 );

----------2------------

 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;