Difference between SQL JOIN and querying from two tables

They are basically equivalent. In general, the JOIN keywords enables you to be more explicit about direction (LEFT, RIGHT) and type (INNER, OUTER, CROSS) of your join.

There is a small difference in syntax, but both queries are doing a join on the P_Id fields of the respective tables.

In your second example, this is an implicit join, which you are constraining in your WHERE clause to the P_Id fields of both tables.

The join is explicit in your first example and the join clause contains the constraint instead of in an additional WHERE clause.

This SO posting has a good explanation of the differences in ANSI SQL complaince, and bears similarities to the question asked here.

While (as it has been stated) both queries will produce the same result, I find that it is always a good idea to explicitly state your JOINs. It's much easier to understand, especially when there are non-JOIN-related evaluations in the WHERE clause.

Explicitly stating your JOIN also prevents you from inadvertently querying a Cartesian product. In your 2nd query above, if you (for whatever reason) forgot to include your WHERE clause, your query would run without JOIN conditions and return a result set of every row in Persons matched with every row in Orders...probably not something that you want.