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