通用表表达式（Common Table Expression）

问题：编写由基本的 SELECT/FROM/WHERE 类型的语句派生而来的复杂 SQL 语句。

方案1：编写在From子句内使用派生表（内联视图）的T-SQL查询语句。

方案2：使用视图

方案3：使用通用表达式（CTE）

比较：

视图：通常用于分解大型查询，无需在临时表中复制或存储数据，数据库中可以重复使用。

 CREATE VIEW vwMyView AS
 SELECT
     EmployeeID, COUNT(*) AS NumOrders,  MAX(OrderDate) AS MaxDate
 FROM Orders
 GROUP BY EmployeeID
 GO

 SELECT
     e.EmployeeID, oe.NumOrders, oe.MaxDate, e.ReportsTo AS ManagerID,
     om.NumOrders, om.MaxDate
 FROM
     Employees AS e
     INNER JOIN vwMyView AS oe ON e.EmployeeID = oe.EmployeeID
     INNER JOIN vwMyView AS om ON e.ReportsTo = om.EmployeeID

派生表：只能在派生表所在的语句中访问它们，使查询变得更难以阅读和维护（若同一个批处理中要多次使用派生表，必须复制粘贴派生表）

 SELECT
     e.EmployeeID, oe.NumOrders, oe.MaxDate,  e.ReportsTo AS ManagerID,
     om.NumOrders, om.MaxDate
 FROM
     Employees AS e
     INNER JOIN
         (SELECT EmployeeID, COUNT(*), MAX(OrderDate)
          FROM Orders
          GROUP BY EmployeeID) AS oe(EmployeeID, NumOrders, MaxDate)
         ON e.EmployeeID = oe.EmployeeID
     LEFT JOIN
         (SELECT EmployeeID, COUNT(*), MAX(OrderDate)
          FROM Orders
          GROUP BY EmployeeID) AS om(EmployeeID, NumOrders, MaxDate)
         ON e.ReportsTo = om.EmployeeID

CET:提升了 T-SQL 的可读性（就像视图一样）,在同一个批处理后紧跟的查询中多次使用,不会在内部创建临时表或虚拟表

 ;WITH EmpOrdersCTE (EmployeeID, NumOrders, MaxDate) AS
 (
   SELECT EmployeeID, COUNT(*), MAX(OrderDate)
   FROM Orders
   GROUP BY EmployeeID
 )

 SELECT
     e.EmployeeID,  oe.NumOrders, oe.MaxDate,
     e.ReportsTo AS ManagerID,  om.NumOrders, om.MaxDate
 FROM
     Employees AS e
     INNER JOIN EmpOrdersCTE oe ON e.EmployeeID = oe.EmployeeID
     LEFT JOIN EmpOrdersCTE om ON e.ReportsTo = om.EmployeeID