ISNULL（电子邮件，“”）=“”为（电子邮件为空或电子邮件=“”）不是由它的自我解释？

Question

为执行一个事实哪一个更好？是否有实际的3个三个版本的SQL服务器（2000/2005/2008）？

之间的差异

Answer 1

您一定要避免使用任何定制或内置功能包的一个在过滤列 - 这严重限制了什么优化器可以在索引使用和seekability方面为你做。你应该使用平等运营商和/或联合方式时可能的习惯，比如这里的情况。将在ISNULL（被非常优选）或聚结（）以下的方法：

where   (
            (t.email is null)
            or
            (t.email = '')
        )

或概述如下能够更好地工作，以及，尝试一下在您的环境，以确定哪个选项最工会的做法。

一个简单的例子将演示的急剧差异可以在性能见：

use tempdb;
go
if object_id('tempdb..#testTable') > 0
    drop table #testTable;
go
-- Build the dataset
select  top 10000000
        cast(cast(a.name as varchar(100)) + '@' + cast(row_number() over (order by a.object_id) as varchar(15)) + '.com' as varchar(150)) as email, 
        row_number() over (order by a.object_id) as id
into    #testTable
from    sys.columns a
cross join sys.columns b
cross join sys.columns c
go
-- Create some nulls
update  #testTable
set     email = null
where   id % 1000 = 0
go
-- Index
create unique clustered index ixc__dbo_testTable__temp__nc1 on #testTable (email,id) on [default];
go
set statistics io on;
set statistics time on;
go
-- Try with isnull - ~cost of about 44.7 on my machine, ~2900ms to execute, and about 49,200 logical reads
select  *
from    #testTable t
where   isnull(t.email,'') = '';
go
-- Try with 'or' - ~cost of about .049 on my machine, ~643ms to execute, about 31 logical reads
select  *
from    #testTable t
where   (
            (t.email is null)
            or
            (t.email = '')
        );
go
-- Try with union approach - ~cost of about .054 on my machine, ~751ms to execute, ~30 logical reads
select  *
from    #testTable t
where   t.email is null
union all
select  *
from    #testTable t
where   t.email = '';
go
if object_id('tempdb..#testTable') > 0
    drop table #testTable;
go

Answer 2

如果你要看到性能上的差异，他们将分。

相信优选的风格是

ISNULL(email, '') = ''