字符串比较因 varchar 变量失败 [英] String comparison fails with varchar variable

问题描述

我无法理解为什么会得到以下结果:

I'm having trouble with understanding why I get the results below:

declare @myVar1 varchar = 'Friday'
declare @myVar2 varchar(10) = 'Friday'

select 
  case when @myVar1 = 'Friday' then 'yes' else 'no' end as test1,
  case when @myVar2 = 'Friday' then 'yes' else 'no' end as test2,
  case when @myVar1 = @myVar2 then 'yes' else 'no' end as test3

我得到的是:

test1: no 
test2: yes 
test3: no

如果声明的 varchar 没有(可选)大小，为什么字符串比较会失败?

Why does the string comparison fail if the varchar is declared without the (optional) size?

推荐答案

答案在这里:http://sqlfiddle.com/#!6/d41d8/4737

declare @myVar1 varchar = 'Friday'
declare @myVar2 varchar(10) = 'Friday'

select len(@myVar1)as len1,
       len(@myVar2)as len2

结果是:

LEN1       LEN2
1           6

因此，如果您没有为 varchar 指定大小，SQL Server 将为您指定.在这种情况下 1. 您应该始终明确指定大小.

So if you don't specify a size for the varchar, SQL Server will do it for you. In this case 1. You should always specify the size explicitly.

要踢的坏习惯:声明没有(长度)的 VARCHAR

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