问题描述
业务需求如下：
有表A，在查询的时候，需要根据标志确定是查询大于某个值的记录，还是小于某个值的记录
A、一般的处理方法
IF @a = 0
Select [TranNumber] FROM A
Where [TranNumber] < 10000
ELSE IF @a = 1
Select [TranNumber] FROM A
Where [TranNumber] > 10000
B、一句的处理方法
Select [TranNumber] FROM A
Where
(@a = 0 AND [TranNumber] < 10000)
or
(@a = 1 AND [TranNumber] > 10000)
分析
从语句的简捷性来看，方法B具有技巧性，它们两者之间，究竟那一个更好呢？你可能会从性能上来评估，以决定到底用那一种。单纯从语句上来看，两者的效率差别应该不会非常大，实际测试的结果会如我们想象吗？继续往下看
建立测试环境（注，此测试环境是为几个主题服务的，因此结构看起来有些怪异）
USE tempdb
GO
SET NOCOUNT ON
--======================================
--创建测试环境
--======================================
RAISERROR('创建测试环境', 10, 1) WITH NOWAIT
-- Table A
Create TABLE [dbo].A(
[TranNumber] [int] IDENTITY(1, 1) NOT NULL,
[INVNO] [char](8) NOT NULL,
[ITEM] [char](15) NULL DEFAULT (''),
PRIMARY KEY([TranNumber])
)
Create INDEX [indexONinvno] ON [dbo].A([INVNO])
Create INDEX [indexOnitem] ON [dbo].A ([ITEM])
Create INDEX [indexONiteminnvo] ON [dbo].A([INVNO], [ITEM])
GO
--======================================
--生成测试数据
--======================================
RAISERROR('生成测试数据', 10, 1) WITH NOWAIT
Insert [dbo].A([INVNO], [ITEM])
Select LEFT(NEWID(), 8), RIGHT(NEWID(), 15)
FROM syscolumns A, syscolumns B
GO
进行性能测试
DECLARE @a int
SET @a = 0
DECLARE @t TABLE(
id int IDENTITY,
a int, b int)
DECLARE @dt datetime, @loop int, @id int
SET @loop = 1
WHILE @loop < 10
BEGIN
SET @loop = @loop + 1
RAISERROR('test %d', 10, 1, @loop) WITH NOWAIT
SET @dt = GETDATE()
IF @a = 0
Select * FROM A
Where [TranNumber] < 10000
ELSE IF @a = 1
Select * FROM A
Where [TranNumber] > 10000
Insert @t(a) VALUES(DATEDIFF(ms, @dt, GETDATE()))
Select @id = SCOPE_IDENTITY(), @dt = GETDATE()
Select * FROM A
Where
(@a = 0 AND [TranNumber] < 10000)
or
(@a = 1 AND [TranNumber] > 10000)
Update @t SET b = DATEDIFF(ms, @dt, GETDATE())
Where id = @id
END
Select * FROM @t
UNION ALL
Select NULL, SUM(a), SUM(b) FROM @t
性能测试结果
id a &nb
