如何使用cross应用Result,可以在sql server中实现使用内部,左,右连接等任何其他连接

Question

这是我的带有示例数据的SQL脚本

CREATE TABLE [dbo].[Employee]
(
    [ID] [INT] IDENTITY(1,1) NOT NULL,
    [Name] [VARCHAR](100) NULL
) ON [PRIMARY]
GO

CREATE TABLE [dbo].[LoginEntry]
(
    [ID] [INT] IDENTITY(1,1) NOT NULL,
    [LoginTime] [DATETIME] NULL,
    [EmpID] [INT] NULL,
    [GateNumber] [VARCHAR](50) NULL
) ON [PRIMARY]
GO

ALTER TABLE Employee 
    ADD CONSTRAINT Pk_Employee PRIMARY KEY (Id)
GO

ALTER TABLE LoginEntry 
    ADD CONSTRAINT Fk_LoginEntry_Employee 
        FOREIGN KEY (EmpId) REFERENCES Employee(Id)
GO

SET IDENTITY_INSERT [dbo].[Employee] ON 
GO

INSERT [dbo].[Employee] ([ID], [Name]) 
VALUES (1, N'Employee 1'), (2, N'Employee 2'), (3, N'Employee 3'),
       (4, N'Employee 4'), (5, N'Employee 5'), (6, N'Employee 6')
GO

SET IDENTITY_INSERT [dbo].[Employee] OFF
GO


SET IDENTITY_INSERT [dbo].[LoginEntry] ON 
GO

INSERT [dbo].[LoginEntry] ([ID], [LoginTime], [EmpID], [GateNumber]) 
VALUES (1, CAST(N'2014-10-24 08:00:00.000' AS DateTime), 1, N'Gate 1'),
       (2, CAST(N'2014-10-24 09:00:00.000' AS DateTime), 1, N'Gate 1'),
       (3, CAST(N'2014-10-24 10:00:00.000' AS DateTime), 1, N'Gate 2'),
       (4, CAST(N'2014-10-24 08:00:00.000' AS DateTime), 2, N'Gate 1'),
       (5, CAST(N'2014-10-24 09:00:00.000' AS DateTime), 2, N'Gate 1'),
       (6, CAST(N'2014-10-24 10:00:00.000' AS DateTime), 2, N'Gate 2'),
       (7, CAST(N'2014-10-24 08:00:00.000' AS DateTime), 3, N'Gate 1'),
       (8, CAST(N'2014-10-24 09:00:00.000' AS DateTime), 3, N'Gate 1'),
       (9, CAST(N'2014-10-24 10:00:00.000' AS DateTime), 3, N'Gate 2'),
       (10, CAST(N'2014-10-24 08:00:00.000' AS DateTime), 4, N'Gate 1'),
       (11, CAST(N'2014-10-24 09:00:00.000' AS DateTime), 4, N'Gate 1'),
       (19, CAST(N'2014-10-24 08:00:00.000' AS DateTime), 5, N'Gate 1'),
       (20, CAST(N'2014-10-24 09:00:00.000' AS DateTime), 5, N'Gate 1'),
       (21, CAST(N'2014-10-24 10:00:00.000' AS DateTime), 5, N'Gate 2'),
       (22, CAST(N'2014-10-24 08:00:00.000' AS DateTime), 6, N'Gate 1'),
       (23, CAST(N'2014-10-24 09:00:00.000' AS DateTime), 6, N'Gate 1'),
       (24, CAST(N'2014-10-24 10:00:00.000' AS DateTime), 6, N'Gate 2')

SET IDENTITY_INSERT [dbo].[LoginEntry] OFF
GO


SELECT 
    e.ID, dt.EmpId, Name, LoginTime
FROM 
    Employee e
CROSS APPLY 
    (SELECT TOP 1
         l.ID, l.LoginTime, l.EmpId
     FROM 
         LoginEntry l 
     WHERE 
         l.EmpId = e.id) dt
GO

结果我得到:

ID  EmpId   Name            LoginTime
-----------------------------------------------
1   1       Employee 1  2014-10-24 08:00:00.000
2   2       Employee 2  2014-10-24 08:00:00.000
3   3       Employee 3  2014-10-24 08:00:00.000
4   4       Employee 4  2014-10-24 08:00:00.000
5   5       Employee 5  2014-10-24 08:00:00.000
6   6       Employee 6  2014-10-24 08:00:00.000

我期待在sql server中使用Joins(内部,右侧,左侧,完整)的相同结果我试过我的运气但不能,请任何人帮助我提前感谢

Answer 1

首先,您的查询不完整.当你TOP 1没有使用时,ORDER BY你永远不会保证会选择哪一个.新数据,并发进程,重新索引,软件补丁,一天中的时间,都会导致结果发生变化.

所以,它应该是......

SELECT
  e.ID,dt.EmpId,Name,LoginTime
FROM
  Employee e
CROSS APPLY
(
  SELECT TOP 1
    l.ID
   ,l.LoginTime
   ,l.EmpId
  FROM
    LoginEntry l
  WHERE
    l.EmpId=e.id
  ORDER BY
    l.LoginTime DESC   -- Will cause TOP 1 to pick the most recent value (per employee)
)
  dt

至于使用连接进行操作,执行TOP 1 (或者greatest-n-per-group,对于您n的连接1),更长,更混乱,更慢.所以我不会参与其中.

但是你可以ROW_NUMBER()用来做TOP 1部分,然后使用a JOIN将结果与你的主表相关联......

WITH
  ordered_logins AS
(
  SELECT
    *,
    ROW_NUMBER() OVER (PARTITION BY EmpID ORDER BY LoginTime DESC, ID DESC) AS row_ordinal
  FROM
    LoginEntry
)
SELECT
  e.ID, l.EmpId, e.Name, l.LoginTime
FROM
  Employee e
LEFT JOIN
  ordered_logins l
    ON  l.EmpID = e.ID
    AND l.row_oridnal = 1

的ROW_NUMBER()受让人的每一行从1周向上的值(每EmpID-分区子句).它是按loginTime递减的顺序,所以最新的登录是第一次,并且只是两个登录具有完全相同的时间,它是由ID desc二次排序.

然后,LEFT JOIN唯一选择编号的行1 (最新登录),如果没有登录则NULL改为s (因此,由于缺少连接,员工记录不会被丢弃).

注意:LEFT JOIN相当于APPLY使用OUTER APPLY而不是CROSS APPLY.