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个人认为CTE最大的做点是可以处理树状存储的数据了

例如类似这样设计的数据表,ID,ParentID这样的设计使用CTE就非常方便,原因就是CTE可以自引用,达到类似递归的效果

那么问题来了,如何使用呢?

想深入学习CTE的看这里

http://www.codeproject.com/Articles/265371/Common-Table-Expressions-CTE-in-SQL-SERVER

https://www.simple-talk.com/sql/t-sql-programming/sql-server-cte-basics/

想直接使用COPY下边的代码,F5看一眼即知,

#1 使用到的测试数据

DECLARE @CTEEXAMPLE TABLE
(
  EmployeeID int NOT NULL PRIMARY KEY,
  FirstName varchar(50) NOT NULL,
  LastName varchar(50) NOT NULL,
  ManagerID int NULL
)

INSERT INTO @CTEEXAMPLE VALUES (101, 'Ken', 'Sánchez', NULL)
INSERT INTO @CTEEXAMPLE VALUES (102, 'Terri', 'Duffy', 101)
INSERT INTO @CTEEXAMPLE VALUES (103, 'Roberto', 'Tamburello', 101)
INSERT INTO @CTEEXAMPLE VALUES (104, 'Rob', 'Walters', 102)
INSERT INTO @CTEEXAMPLE VALUES (105, 'Gail', 'Erickson', 102)
INSERT INTO @CTEEXAMPLE VALUES (106, 'Jossef', 'Goldberg', 103)
INSERT INTO @CTEEXAMPLE VALUES (107, 'Dylan', 'Miller', 103)
INSERT INTO @CTEEXAMPLE VALUES (108, 'Diane', 'Margheim', 105)
INSERT INTO @CTEEXAMPLE VALUES (109, 'Gigi', 'Matthew', 105)
INSERT INTO @CTEEXAMPLE VALUES (110, 'Michael', 'Raheem', 106);

#2 CTE递归查询

DECLARE @CTEEXAMPLE TABLE
(
  EmployeeID int NOT NULL PRIMARY KEY,
  FirstName varchar(50) NOT NULL,
  LastName varchar(50) NOT NULL,
  ManagerID int NULL
)

INSERT INTO @CTEEXAMPLE VALUES (101, 'Ken', 'Sánchez', NULL)
INSERT INTO @CTEEXAMPLE VALUES (102, 'Terri', 'Duffy', 101)
INSERT INTO @CTEEXAMPLE VALUES (103, 'Roberto', 'Tamburello', 101)
INSERT INTO @CTEEXAMPLE VALUES (104, 'Rob', 'Walters', 102)
INSERT INTO @CTEEXAMPLE VALUES (105, 'Gail', 'Erickson', 102)
INSERT INTO @CTEEXAMPLE VALUES (106, 'Jossef', 'Goldberg', 103)
INSERT INTO @CTEEXAMPLE VALUES (107, 'Dylan', 'Miller', 103)
INSERT INTO @CTEEXAMPLE VALUES (108, 'Diane', 'Margheim', 105)
INSERT INTO @CTEEXAMPLE VALUES (109, 'Gigi', 'Matthew', 105)
INSERT INTO @CTEEXAMPLE VALUES (110, 'Michael', 'Raheem', 106);

WITH cteReports (EmpID, FirstName, LastName, MgrID, EmpLevel)
AS
  (
    SELECT EmployeeID, FirstName, LastName, ManagerID, 1
    FROM @CTEEXAMPLE
    WHERE ManagerID IS NULL
    UNION ALL
    SELECT e.EmployeeID, e.FirstName, e.LastName, e.ManagerID,
      r.EmpLevel + 1
    FROM @CTEEXAMPLE e
      INNER JOIN cteReports r ON e.ManagerID = r.EmpID
  )
SELECT
    R.EmpID,R.FirstName + R.LastName 'fullName',R.MgrID,R.EmpLevel,
    'Employee<----|---->Manager',
    C.FirstName + C.LastName 'fullName'
FROM cteReports R
    LEFT JOIN @CTEEXAMPLE C ON R.MgrID = C.EmployeeID
ORDER BY EmpLevel, MgrID