UNION does not set any relations between queries, it only combines there results. I'm not sure why you would expect anything else.

>I recently crashed when I tried:
>
>
>select intcolumn, charcolumn from tablea
>union all
>select charcolumn, intcolumn from tableb
>The message was that the data types couldn't be converted.
>
>Checking SQL help, I was surprised to learn that both the number of columns and the order of the columns have to match- column names don't seem to matter at all.
>
>
>So.. this UNION of all char columns doesn't crash :
>
>
>select sealnumber, customer from tablea
>union all
>select customer,sealnumber from tableb
>
>but the result is whacky.
>The sealnumber column contains sealnumbers from table A and customers from table B and
>the customer column contains customers from table A and sealnumbers from table B