您可以为公用表表达式创建嵌套的WITH子句吗？

Question

WITH y AS (
    WITH x AS (
        SELECT * FROM MyTable
    )
    SELECT * FROM x
)
SELECT * FROM y

这样的事情有用吗？我之前尝试过,但我无法让它发挥作用.

Answer 1

虽然没有严格嵌套,但您可以使用公用表表达式重用以后的查询.

为此,您要查找的语句的形式将是

WITH x AS 
(
    SELECT * FROM MyTable
), 
y AS 
(
    SELECT * FROM x
)
SELECT * FROM y

Answer 2

您可以执行以下操作,称为递归查询:

WITH y
AS
(
  SELECT x, y, z
  FROM MyTable
  WHERE [base_condition]

  UNION ALL

  SELECT x, y, z
  FROM MyTable M
  INNER JOIN y ON M.[some_other_condition] = y.[some_other_condition]
)
SELECT *
FROM y

您可能不需要此功能.为了更好地组织我的查询,我已经完成了以下操作:

WITH y 
AS
(
  SELECT * 
  FROM MyTable
  WHERE [base_condition]
),
x
AS
(
  SELECT * 
  FROM y
  WHERE [something_else]
)
SELECT * 
FROM x

Answer 3

与嵌入不起作用,但它确实连续工作

;WITH A AS(
...
),
B AS(
...
)
SELECT *
FROM A
UNION ALL
SELECT *
FROM B

编辑 修正了语法...

另外,请看下面的示例

SQLFiddle DEMO