ISNULL()

The ISNULL  function in SQL Server is used to substitute alternate value if the one being checked is “NULL”. 
Take the following example.

DECLARE @var1 VARCHAR(50) 
DECLARE @var2 VARCHAR(5) 

SET @var1 = 'Hello'
SET @var2= NULL

SELECT ISNULL(@var2,@var1)

Your output is Hello

Now try substituting @var1 with a bigger sentence

SET @var1 = 'Hello World Its great to be here'

Now your output is just Hello


This is because the ISNULL funnction type casts var2 to var1
Try the following, it will resolve the issue

SELECT ISNULL(CONVERT(VARCHAR(50), @var2),@var1)

http://msdn.microsoft.com/en-us/library/aa933210(v=sql.80).aspx