Question

有一个结果集，其中所有记录都是这样的唯一列。如果存在属于同一BusinessID的另一条记录，我只想显示具有较新日期的记录。我得到这个结果的查询是

SELECT 
  FirstName, 
  LastName,
  Department,
  StartDate,
  Title,
  PhoneNumber,
  BusinessEntityID
FROM
  (SELECT
     E.BusinessEntityID,
     H.FirstName,
     H.LastName,
     H.Title,
     H.Department,
     E.PhoneNumber,
     E.CountryRegionName,
     E.PostalCode,
     H.StartDate
   FROM 
     CS120Exam_EmployeeDepartmentHistory H
   JOIN 
     CS120Exam_Employee E
   ON 
     E.BusinessEntityID=H.BusinessEntityID ) x 
   ORDER BY BusinessEntityID

结果是

BusinessID  FirstName LastName Department StartDate
----------- --------- -------- ---------- ----------
1           aaa       mate     staff      2002-02-02 <----- DO NOT want this 
1           aaa       mate     admin      2004-03-05
2           john      mate     admin      2001-03-06
3           sun       kent     admin      2004-03-05
4           bbb       clark    staff      2006-02-02 <----- DO NOT want this 
4           bbb       clark    admin      2009-03-05

Answer 1

您需要一个子选择来查找每个BusinessEntityID的最新日期。试试这个（当然未经测试）：

SELECT 
  FirstName, 
  LastName,
  Department,
  StartDate,
  Title,
  PhoneNumber,
  BusinessEntityID
FROM
  (SELECT
     E.BusinessEntityID,
     H.FirstName,
     H.LastName,
     H.Title,
     H.Department,
     E.PhoneNumber,
     E.CountryRegionName,
     E.PostalCode,
     H.StartDate
   FROM 
     CS120Exam_EmployeeDepartmentHistory H
   JOIN 
     CS120Exam_Employee E
   ON 
     E.BusinessEntityID=H.BusinessEntityID ) x 
   WHERE x.StartDate = 
     (SELECT 
        Max(cs.StartDate) 
      FROM
        CS120Exam_EmployeeDepartmentHistory cs
      WHERE
        cs.BusinessEntityID = x.BusinessEntityID)
   ORDER BY BusinessEntityID

Answer 2

您可以使用GROUP BY和max（startdate）

SELECT 
  FirstName,
  LastName,
  Department,
  max(StartDate),
  Title,
  PhoneNumber,
  BusinessEntityID
FROM
  (SELECT 
     E.BusinessEntityID,
     H.FirstName,
     H.LastName,
     H.Title,
     H.Department,
     E.PhoneNumber,
     E.CountryRegionName,
     E.PostalCode,
     H.StartDate
  FROM 
    CS120Exam_EmployeeDepartmentHistory H
  JOIN 
    CS120Exam_Employee E
  ON 
   E.BusinessEntityID=H.BusinessEntityID ) x 
  ORDER BY 
    BusinessEntityID
  GROUP BY 
    FirstName,
    LastName,
    Department,
    Title,
    PhoneNumber,
    BusinessEntityID

Answer 3

我正在考虑按其余列进行分组并获取最大日期。在这种情况下，您将获得具有最大日期

的单个记录

SELECT FirstName,LastName,Department,max(StartDate),Title,PhoneNumber,BusinessEntityID
FROM
(SELECT E.BusinessEntityID,H.FirstName,H.LastName,H.Title,H.Department,E.PhoneNumber,
E.CountryRegionName,E.PostalCode,H.StartDate
FROM CS120Exam_EmployeeDepartmentHistory H
JOIN CS120Exam_Employee E
ON E.BusinessEntityID=H.BusinessEntityID )x 
group by  FirstName,LastName,Department,Title,PhoneNumber,BusinessEntityID
ORDER BY BusinessEntityID

Answer 4

SELECT 
  FirstName, 
  LastName,
  Department,
  StartDate,
  Title,
  PhoneNumber,
  BusinessEntityID
FROM
  (SELECT
     E.BusinessEntityID,
     H.FirstName,
     H.LastName,
     H.Title,
     H.Department,
     E.PhoneNumber,
     E.CountryRegionName,
     E.PostalCode,
     H.StartDate,
     row_number() over (partition by BusinessID order by StartDate desc) as rn
   FROM 
     CS120Exam_EmployeeDepartmentHistory H
   JOIN 
     CS120Exam_Employee E
   ON 
     E.BusinessEntityID=H.BusinessEntityID ) x 
WHERE rn = 1
ORDER BY BusinessEntityID

可能比使用子选择更快获得最大值。 startdate因为只需要对表进行一次扫描。

摆脱旧记录

4 个答案: