Remove all Null values from Sql table at once
I'm not sure I completely understand what you are trying to do. ISNULL(COLUMNNAME,0) is a function that returns 0 if COLUMNNAME contains a null (it does not change anything in the table, you will have to do an update to modify the table). 

The error message you get is likely because COLUMNNAME is a different type than a number. If COLUMNNAME is of type string you can use:

ISNULL(COLUMNNAME, '')


if it's of type date you need a date:

ISNULL(COLUMNNAME, DATE('1900-01-01') ) -- I just invented one here, use one that makes sense in your context


and so forth. As you noted, applying a function to a column does an implicit rename of the resulting column. Most DBMS generates a rather cryptic name so it makes sense to do an explicit rename as in:

ISNULL(COLUMNNAME, '') AS COLUMNNAME


if you want to preserve the name.