1 CTE语法WITH关键字
通用表表达式（Common Table Express， CTE），将派生表定义在查询的最前面。要使用CTE开始创建一个查询，可以使用WITH关键字。
CTE语法:
WITH <expression_name> [(column_name [,...n])]
AS
( CTE_query_definition)
[, <another_expression>]
<query>
首先为CTE提供一个名称，该名称类似于用于派生表的别名。然后可以提供CTE将返回的列名列表；如果CTE指定了它的所有返回列，则这是可选操作。最后，在圆括号中添加CTE查询的定义，最后添加使用CTE的主查询。
WITH关键字之前的语句必须使用分号(;)结束。
示例
WITH cte
AS
(
SELECT * FROM [dbo].[Product]
) SELECT [ProductID],[ProductCode],[ProductName],[UnitPrice] FROM cte
连接查询示例
WITH ProductCTE([CategoryID], [ProductsCount])
AS
(
SELECT [CategoryID],COUNT(1)
FROM [dbo].[Product]
GROUP BY [CategoryID]
) SELECT c.[CategoryID],c.[CategoryName], cte.[ProductsCount]
FROM [dbo].[Category] c
INNER JOIN ProductCTE cte
ON c.[CategoryID] = cte.[CategoryID]
ORDER BY cte.[ProductsCount]
2 使用多个CTE
使用WITH开始语句可以定义多个CTE，不需要重复使用WITH关键字，每一个CTE可以使用在该语句中已经定义的任意CTE（作为其定义的一部分）。
WITH CategoryCTE
AS
(
SELECT * FROM [dbo].[Category]
),ProductCTE
AS
(
SELECT p.*,cte.[CategoryName] FROM [dbo].[Product] p
INNER JOIN CategoryCTE cte
ON p.[CategoryID] = cte.[CategoryID]
) SELECT * FROM ProductCTE
3 递归CTE
递归公用表表达式是在CTE内的语句中调用其自身的CTE。
示例
WITH cte([CategoryID],[CategoryName],[ParentID],[Level])
AS
(
-- 查询语句
SELECT [CategoryID],[CategoryName],[ParentID],1 AS [Level] FROM [dbo].[Category]
WHERE [ParentID] IS NULL
UNION ALL
-- 递归语句
SELECT c.[CategoryID],c.[CategoryName],c.[ParentID], cte.[Level] + 1
FROM [dbo].[Category] c
INNER JOIN cte
ON c.[CategoryID] = cte.[ParentID]
) SELECT [CategoryID],[CategoryName],[ParentID],[Level] FROM cte
限制递归层次
WITH cte([CategoryID],[CategoryName],[ParentID],[Level])
AS
(
-- 查询语句
SELECT [CategoryID],[CategoryName],[ParentID],1 AS [Level] FROM [dbo].[Category]
WHERE [ParentID] IS NULL
UNION ALL
-- 递归语句
SELECT c.[CategoryID],c.[CategoryName],c.[ParentID], cte.[Level] + 1
FROM [dbo].[Category] c
INNER JOIN cte
ON c.[CategoryID] = cte.[ParentID]
) SELECT [CategoryID],[CategoryName],[ParentID],[Level] FROM cte
OPTION(MAXRECURSION 2)
Where过滤递归结果数据层次
WITH cte([CategoryID],[CategoryName],[ParentID],[Level])
AS
(
-- 查询语句
SELECT [CategoryID],[CategoryName],[ParentID],1 FROM [dbo].[Category]
WHERE [ParentID] IS NULL
UNION ALL
-- 递归语句
SELECT c.[CategoryID],c.[CategoryName],c.[ParentID], [Level] + 1
FROM [dbo].[Category] c
INNER JOIN cte
ON c.[CategoryID] = cte.[ParentID]
) SELECT [CategoryID],[CategoryName],[ParentID],[Level] FROM cte
WHERE cte.[Level] <= 3