SQL 'Select AS' syntax

I have an SQL statement where I select against a single file and use the AS keyword and give it an ID. For example

Select field1, field2 from file1 as tbl

Then I use tbl as a reference in a later sub select statement. As was well.

Now I need to change the initial select to select values from two files and it no longer likes the AS. For example:

Select field1, field2, field3 from File1, File2 where key1 = key2 AS tbl

I get the error:

Keyword AS not expected. Valid tokens: FOR WITH FETCH ORDER UNION ...

I tried adding parenthesis and a few other syntax changes but I get the same error.

So, how can I use the "as" keyword when my select uses more than one file?

Thanks,

Pete