Question

我正在尝试从三个单独的表中获取最新日期，每个表都有一个“ lastmodified”列。本质上，我正在尝试计算数据库的上次修改时间。

例如：

Table 1 -lastmodified column
Table 2 -lastmodified column
Table 3 -lastmodified column

我正在捆绑以下代码。我知道从每个表中获取最新最新记录的部分是错误的，但是我不确定如何更改它以使其正确。

(SELECT MAX(v) FROM (VALUES 
    (SELECT MAX(lastmodifieddate) FROM IMAGES),
    (SELECT MAX(lastmodifieddate) FROM VIDEOS),
    (SELECT MAX(lastmodifieddate) FROM DOCUMENTS)
) AS VALUE (v)) AS MaxDate)

谢谢。

Answer 1

您需要使用UNION将记录合并到来自不同表的一列中，然后从其中获取MAX。

SELECT MAX(lastmodifieddate) AS lastmodifieddate
FROM
(
SELECT MAX(lastmodifieddate) AS lastmodifieddate FROM IMAGES
UNION 
SELECT MAX(lastmodifieddate) FROM VIDEOS
UNION
SELECT MAX(lastmodifieddate) FROM DOCUMENTS
) T

Answer 2

使用表中的union合并3个表中的3个最大日期，然后获取该子查询的max()

SELECT max(date_m) as  maxdateof3tables from
(
  select MAX(lastmodifieddate) as date_m FROM IMAGES
  union     
  SELECT MAX(lastmodifieddate) FROM VIDEOS
  union
  SELECT MAX(lastmodifieddate) FROM DOCUMENTS
 ) as t

Answer 3

SELECT MAX(lastmodifieddate)
FROM (
SELECT lastmodifieddate from IMAGES
UNION ALL
SELECT lastmodifieddate from VIDEOS
UNION ALL
SELECT lastmodifieddate from DOCUMENTS
)T

Answer 4

您Max与Union All一起使用

SELECT MAX(t) AS lastmodifieddate
FROM
(
SELECT MAX(lastmodifieddate) AS t FROM IMAGES
UNION ALL
SELECT MAX(lastmodifieddate) FROM VIDEOS
UNION ALL
SELECT MAX(lastmodifieddate) FROM DOCUMENTS
) Temp

从3个表格中获取最新日期

4 个答案: