Question

If you add more than one table to the FROM clause (in a query), how does this impact the result set? Does it first select from the first table then from the second and then create a union (i.e., only the rowspace is impacted?) or does it actually do something like a join (i.e., extend the column space)? And when you use multiple tables in the FROM clause, does the WHERE clause filter both sub-result-sets?

Answer 1

Specifying two tables in your FROM clause will execute a JOIN. You can then use the WHERE clause to specify your JOIN conditions. If you fail to do this, you will end-up with a Cartesian product (every row in the first table indiscriminately joined to every row in the second).

The code will look something like this:

SELECT a.*, b.*
FROM table1 a, table2 b
WHERE a.id = b.id

However, I always try to explicitly specify my JOINs (with JOIN and ON keywords). That makes it abundantly clear (for the next developer) as to what you’re trying to do. Here’s the same JOIN, but explicitly specified:

SELECT a.*, b.*
FROM table1 a
INNER JOIN table2 b ON b.id = a.id

Note that now I don’t need a WHERE clause. This method also helps you avoid generating an inadvertent Cartesian product (if you happen to forget your WHERE clause), because the ON is specified explicitly.