Beginning SQL

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
You might argue that if your database has only six categories, looking up each category is not that hard or
time-consuming. It’s a different story altogether, though, if your database contains 50 or 100 categories.
Also, while computers might be more at home with numbers, most humans prefer names. For example,
imagine that you create a film club Web site that contains a page allowing users to choose a category
and then display all the films for that category. It’s unlikely that the Web site user would want to choose
categories based on category IDs. What’s more likely is that users would choose by name and allow the
database to work out the ID and display the results.
That said, how can you use a join to obtain a list of films in the Historical category?
First, you need to determine which table contains category names and allows you to look up the
CategoryId. From the previous example, it’s clearly the Category table that provides this information.
Second, you need to determine which table or tables provide the results you want. Again, based on the
preceding example, you know that the Films table contains the data needed. The task now is to join
the two tables together. To join the tables, you need to find a link between the two tables. No prizes for
guessing that the CategoryId field, which is present in both tables, links the two!
The type of join to use in this case is an inner join. An inner join combines two tables and links, or joins,
them based on columns within the tables. The inner join allows you to specify which columns form the
join and under what conditions. For example, you could specify a condition that says the MemberId column
in the MemberDetails table matches a value from the MemberId column in the FavCategory table.
Then only records where there is a matching MemberId in both tables are included in the results. To create
an inner join, you must specify the two tables to be joined and the column or columns on which the join is
based. The syntax looks like this:
table1 INNER JOIN table2 ON column_from_table1 = column_from_table2
Applying the syntax to the problem at hand yields the following code:
SELECT FilmName, YearReleased, Rating
FROM Films INNER JOIN Category
ON Films.CategoryId = Category.CategoryId
WHERE Category.CategoryId = 6;
Extracting Information
At the top is the SELECT statement’s list of the columns required to form the results. On the following
line are the tables from which the results are drawn. Specify them as normal, except this time the INNER
JOIN keyword specifies that the two tables should be joined. The ON keyword that follows specifies what
joins the tables; in this case, the CategoryId field joins them. Note that you must specify the table names
in front of the CategoryId, otherwise the database doesn’t know if you mean the CategoryId in the Films
table or the CategoryId in the Category table. In fact, the two columns don’t actually need to have the
same name, depending on the database’s design. Designing a database that way doesn’t make sense,
though; when someone looks at the database design, they should be able to see that CategoryId in each
table relates to the same set of values.
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