isNull(email,“)=”不是由它自己解释为(email为空或email='')?

你肯定想避免使用任何 custom or built-in functions wrapping a column in a filter -它严重限制了优化器在索引使用和可查看性方面可以为您做什么。您应该养成尽可能使用相等运算符和/或联合方法的习惯,这里就是这样。与isNull()或coalesce()方法相比,以下方法更为可取:

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

或者下面概述的联合方法也可以更好地工作,在您的环境中进行尝试,以确定哪个选项是最好的。

一个简单的例子将展示您在性能上看到的巨大差异:

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

如果你想看到性能的差异,它们会很快出现。

我认为首选的款式是

ISNULL(email, '') = ''