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