Question

我在从UNION子查询获取最大日期时遇到麻烦。我的代码如下：

SELECT MX.LOCATION AS LOC, MAX(MX.TIMESTAMP) AS TIME
FROM

  (SELECT TAB1.LOCATION AS LOCATION, TAB2.TIMESTAMP AS TIMESTAMP
   FROM TABLE1 TAB1
   JOIN TABLE2 TAB2 ON TAB1.ID=TAB2.ID
   WHERE TAB2.COMPANY =3

   UNION

   SELECT TAB3.LOCATION AS LOCATION, TAB2.TIMESTAMP AS TIMESTAMP
   FROM TABLE3 TAB3
   WHERE TAB3.COMPANY = 3) MX

GROUP BY MX.LOCATION

在两个由UNION连接的查询中具有相同位置时，会出现问题。我得到的是来自第一个查询和第二个查询的MAX时间戳。我想要的只是每个位置的MAX-而不是两个。

样本数据。联盟声明的输出。表列仅表示数据集行不正确

LOCATION  TIMESTAMP        TABLE
855       2017-07-29 13:48 TAB1
856       2017-07-28 14:50 TAB1
855       2017-07-29 11:48 TAB2

RESULT
LOCATION  TIMESTAMP
855       2017-07-29 11:48
855       2017-07-29 13:48
856       2017-07-28 14:50

Answer 1

问题解决了。一些位置中有空格。我用TRIM函数来解决它。

Answer 2

查询可以写在下面

with allTableInformation as (

SELECT tb1.location, tb1.time
from table1 tb1

union
select tb2.location, tb2.time
from table2 tb2

union
select tb3.location, tb3.time
from table3 tb3
) 
select distinct on (location) location, time from allTableInformation 
order by location, time desc

您可以尝试在demo

中查询

带有UNION子查询的MAX无法产生期望的结果

2 个答案: