String comparison fails with varchar variable

Question

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

What I get is:

test1: no 
test2: yes 
test3: no

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

Answer 1

Here's the answer: http://sqlfiddle.com/#!6/d41d8/4737

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

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

Result is:

LEN1       LEN2
1           6

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.

Bad habits to kick : declaring VARCHAR without (length)