從3個表格中獲取最新日期

Question

我試圖從三個單獨的表中獲取最新的日期，每個表都有一個“ lastmodified”列。 本質上，我正在嘗試計算數據庫的上次修改時間。

例如：

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

我正在綁下面的代碼。 我知道從每個表中獲取最新最新記錄的那部分是錯誤的，但是我不確定如何更改它以使其正確。

(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)

先感謝您。

Answer 1

您需要使用UNION將記錄合並到來自不同表的一列中，然后再獲取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

在表之間使用union以合並3個表中的3個最大日期，然后采用該子查詢的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 Union All功能

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