Difference between SQL JOIN and querying from two tables

Question

What is the difference between the query

SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo
FROM Persons
INNER JOIN Orders
ON Persons.P_Id=Orders.P_Id
ORDER BY Persons.LastName

and this one

SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo
FROM Persons, Orders
WHERE Persons.P_Id=Orders.P_Id
ORDER BY Persons.LastName

Answer 1

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.

Answer 2

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.