Question

Okay the title cannot explain the situation correct enough.
Now this is it,
I have a table with columns

Table1
Columns0, CHAR(20), NOT NULL.
Columns1, CHAR(4), Allow Nulls.
Data : 'ARR '

Table2
Columns0, CHAR(20), NOT NULL.
Columns1, CHAR(4), NOT NULL.
Data : 'ARR '

Then I join two tables together.

SELECT (ISNULL(a.Columns1,'') + ISNULL(b.Columns1,'')) AS WhatEver
FROM Table1 a 
left join Table2 b on a.Columns0 = b.Columns0

The result should appear as 'ARR ARR '
instead of this, it appear as 'ARRARR '

Why does this happen?

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EDIT 2012/06/11:

After struggling, I ended up doing things like:

SELECT ISNULL(CONVERT(CHAR(4),a.Columns1),'') + ISNULL(b.Columns1,'')

Then only I get correct result 'ARR ARR '

But I am pretty sure my data type is CHAR(4) in the database.

Thank guys..

---

EDIT 2012/08/06:

Another solution that I found working is to change the TABLE 2 COLUMN 1 to (CHAR(4), NULL).
This can be done through
ALTER TABLE [table_name] MODIFY [column_name] [column_data_type] [null|not null]

Answer 1

Your example works fine on my server (SQL Server 2008 R2):

create table SOxx1
(
    col1  char(4),
    col2  char(20) not null
)

create table SOxx2
(
    col1  char(4),
    col2  char(20) not null
)
Go

insert into SOxx1 (col1, col2) VALUES ('ARR ', 'abc')
insert into SOxx2 (col1, col2) VALUES ('ARR ', 'abc')
go

SELECT (ISNULL(a.col1,'') + ISNULL(b.col1,'')) AS WhatEver 
FROM SOxx1 a  
left join SOxx2 b on a.Col2 = b.Col2

-- OUTPUT is 'ARR ARR '