Question

我有一个无法更改的查询。例如：

SELECT studentid FROM school s LEFT OUTER JOIN mall m ON s.studentid=m.studentid

学生在购物中心有两排。上面的查询输出重复项，因为学生在表商城上有两行具有相同ID的行。表商城上有一个日期列，我只想选择具有最新日期的学生。我该怎么办？（只是在where子句中添加条件）

Answer 1

where m.date=(select max(date) from mall where studentid=m.studentid)

Answer 2

试试这个：

select s.studentid 
from school s 
INNER JOIN malls m1 ON m1.studentid = s.studentid
INNER JOIN
(
   SELECT studentid, Max(DateField) MaxDate 
   FROM mall
   GROUP BY studentid
) m2 on m1.studentid = m2.studentid AND m1.DateField = m2.MaxDate

Answer 3

SELECT studentid 
FROM school s 
LEFT OUTER JOIN mall m 
ON s.studentid=m.studentid
ORDER BY date DESC
GROUP BY studentid

Answer 4

你可以使用order by和limit如下：

select studentid 
from school s 
left outer join mall m on s.studentid=m.studentid 
order by date desc 
limit 1

这比在where子句上使用多个select查询更简单。

Answer 5

如果您使用SQL Server

WITH LatestRecord
AS
(
    SELECT  studentID, date,
            ROW_NUMBER() OVER(Partition BY StudentID Order By DATE DESC) AS RN
    FROM    mall
)
SELECT  a.*, b.studentID, b.date
FROM    School a
        INNER JOIN LatestRecord b
            on a.StudentID = b.Student_ID
WHERE   RN = 1

选择有多行的1行？

5 个答案: