SqlServer 2005 T-SQL Query 学习笔记（4）

作者他很喜欢建立数字辅助表（即是1-N的数字按顺序组成的表），关于如何建立这些辅助表，然后他给了一些例子，这些例子很有代表性。

 

比如，我要建立一个1，000，000行的数字表：

CREATE TABLE dbo.Nums(n INT NOT NULL PRIMARY KEY);
DECLARE @max AS INT, @rc AS INT;
SET @max = 1000000;
SET @rc = 1;

INSERT INTO Nums VALUES(1);
WHILE @rc * 2 <= @max
BEGIN
  INSERT INTO dbo.Nums SELECT n + @rc FROM dbo.Nums;
  SET @rc = @rc * 2;
END

INSERT INTO dbo.Nums
  SELECT n + @rc FROM dbo.Nums WHERE n + @rc <= @max;

 

这种方式非常巧妙，它并不是一个一个的循环插入，而是一次插入很多行，{1}，{2}，{3，4}，{5，6，7，8}。。。

 

为什么这样会快呢？

是因为它节省了跟比较其他可用解决方案和记录这些日志的时间。

 

然后，作者给了一个CTE的递归的解决方案:

DECLARE @n AS BIGINT;
SET @n = 1000000;

WITH Nums AS
(
  SELECT 1 AS n
  UNION ALL
  SELECT n + 1 FROM Nums WHERE n < @n
)
SELECT n FROM Nums
OPTION(MAXRECURSION 0);--为了移除默认100的递归限制

有个更优的CTE的解决方案，就是先生成很多行，然后用ROW_NUMBER进行计算，再选择ROW_NUMBER这列的值就可以了。

DECLARE @n AS BIGINT;
SET @n = 1000000;

WITH Base AS
(
  SELECT 1 AS n
  UNION ALL
  SELECT n + 1 FROM Base WHERE n < CEILING(SQRT(@n))
),
Expand AS
(
  SELECT 1 AS c
  FROM Base AS B1, Base AS B2
),
Nums AS
(
  SELECT ROW_NUMBER() OVER(ORDER BY c) AS n
  FROM Expand
)
SELECT n FROM Nums WHERE n <= @n
OPTION(MAXRECURSION 0);

利用笛卡尔积进行不断的累加，达到了22n行。

最后，作者给出了一个函数，用于生成这样的数字表：

CREATE FUNCTION dbo.fn_nums(@n AS BIGINT) RETURNS TABLE
AS
RETURN
  WITH
  L0   AS(SELECT 1 AS c UNION ALL SELECT 1),
  L1   AS(SELECT 1 AS c FROM L0 AS A, L0 AS B),
  L2   AS(SELECT 1 AS c FROM L1 AS A, L1 AS B),
  L3   AS(SELECT 1 AS c FROM L2 AS A, L2 AS B),
  L4   AS(SELECT 1 AS c FROM L3 AS A, L3 AS B),
  L5   AS(SELECT 1 AS c FROM L4 AS A, L4 AS B),
  Nums AS(SELECT ROW_NUMBER() OVER(ORDER BY c) AS n FROM L5)
  SELECT n FROM Nums WHERE n <= @n;
GO

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