Beginning SQL

Chapter 3
92
The preceding SQL produces the same results as before:
FilmName YearReleased Rating
Sense and Insensitivity 2001 3
15th Late Afternoon 1989 5
Gone with the Window Cleaner 1988 3
The Good, the Bad, and the Facially Challenged 1989 5
The INNER part of INNER JOIN is actually optional in most database systems: INNER JOIN is the default
join because it’s the most common. That said, you can write the SQL as follows:
SELECT FilmName, YearReleased, Rating
FROM Films JOIN Category
ON Films.CategoryId = Category.CategoryId
WHERE Category.CategoryId = 6;
Using INNER JOIN or simply JOIN to create an inner join between tables is not, in fact, the only way to
join tables. You may prefer it, though, because INNER JOIN and JOIN make explicit which tables are
being joined, and that in turn makes the SQL easier to read and understand. The alternative way of creating
an inner join is simply to specify the link in the WHERE clause. Rewriting the preceding SQL by
specifying the link looks like this:
SELECT FilmName, YearReleased, Rating
FROM Films, Category
WHERE Films.CategoryId = Category.CategoryId AND
Category.CategoryId = 6;
The WHERE clause specifies that Films.CategoryId should equal Category.CategoryId, which creates the
join.
So far, you’ve used the equals operator (=) to join tables, which is termed equijoin. Equijoin is the most
common join type, but using any of the other operators is fine.
As mentioned earlier, you’re not limited to joining just two tables together in one SELECT statement;
within reason, it’s possible to join as many tables as you like. You are sure to encounter a problem that
requires joining multiple tables. For example, say you want to produce a list of each film club member’s
name and all the films they enjoy based on their favorite film category. In the results, you want to display
the members’ first and last names, the name of each film, each film’s year of release, and finally the
category in which each film belongs.
Now this might seem like a fairly simple problem, but it actually involves the most complex SQL so far
in the book. When illustrated step-by-step, however, it’s not so bad at all.
Having a diagram of the database tables and their fields often helps when it comes to the more tricky
SQL queries. Shown in Figure 3-1 is a diagram of the Film Club database.