Question

我有4个表格（比如A，B，C和D），所有这些表都带有＆＃39; date＆＃39;。我需要在所有四个表中找到最大的公共日期值。也就是说，所有四个表中存在的最大日期值。我怎么能这样做？

目前，我正在查找所有四个表的MAX日期值的MIN，但是如果MIN存在于一个表中但不存在于第二个表中，则会失败。

这是一个让事情更清晰的例子：

A.date
------
2015-03-31
2015-03-30
2015-03-29
2015-03-27

B.date
------
2015-03-30
2015-03-29
2015-03-28
2015-03-27

C.date
------
2015-03-29
2015-03-27
2015-03-26
2015-03-25

D.date
------
2015-03-28
2015-03-27
2015-03-26
2015-03-25

我为找到最高共同日期所做的是：

SELECT MIN(max_date) FROM (
SELECT MAX(date) AS max_date FROM A
UNION
SELECT MAX(date) AS max_date FROM B
UNION
SELECT MAX(date) AS max_date FROM C
UNION
SELECT MAX(date) AS max_date FROM D
) T;

这给了我2015-03-28，但后来我意识到有些表可能没有这个日期。我真正想要的日期是2015-03-27。

Answer 1

这是一种方法：

select date
from (select date, 'a' as which from a union all
      select date, 'b' as which from b union all
      select date, 'c' as which from c union all
      select date, 'd' as which from d
     ) x
group by date
having count(distinct which) = 4
order by date desc
limit 1;

以下版本可能会更好一些，特别是如果每个表中的date都有索引：

select date
from (select distinct date, 'a' as which from a union all
      select distinct date, 'b' as which from b union all
      select distinct date, 'c' as which from c union all
      select distinct date, 'd' as which from d
     ) x
group by date
having count(*) = 4
order by date desc
limit 1;

Answer 2

您需要在4个单独的表中获得所有日期值的交集。然后，选择这些值的MAX：

SELECT MAX(date)
FROM A
WHERE date IN (
  SELECT date 
  FROM B
  WHERE date IN (
     SELECT date
     FROM C
     WHERE date IN (
        SELECT date
        FROM D)))

SQL Fiddle Demo here

跨表的列中获取最大的公共值

2 个答案: