Beginning SQL

4. Next, quickly double-check a few of the results to see whether they really are correct. Having
confirmed that they are, you can add the INSERT INTO bit:
INSERT INTO FavCategory (CategoryId, MemberId)
SELECT 9, MemberId FROM MemberDetails AS MD1
WHERE EXISTS
(SELECT * from FavCategory FC1
WHERE FC1.CategoryId = 1 AND FC1.MemberId = MD1.MemberId
AND NOT EXISTS
(SELECT * FROM FavCategory AS FC2
WHERE FC2.MemberId = FC1.MemberId AND
FC2.CategoryId = 9));
How It Works
The INSERT INTO inserts the literal value 9 (representing the Film Noir’s CategoryId) and
MemberId into the FavCategory table. When you execute the SQL, you should find that four
rows are added. Execute it a second, a third, or however many times, and no more rows will be
added as the query checks for duplication, a good safeguard.
In the first step, you selected 9, or the CategoryId of the Thriller category, and also MemberId from the
MemberDetails table.
In the next step, you added a WHERE clause that uses a subquery. Using the EXISTS operator, it checks to
see whether there are any rows when selecting records from the FavCategory table where the MemberId
is the same as that in the MemberDetails table and where CategoryId is equal to 1 (the Thriller ID).
Using FC1.MemberId = MD1.MemberId joins MemberDetails and FavCategory, making sure that the
current rows in the MemberDetails and FavCategory tables are the same; otherwise the subquery would
just return rows where any member had selected a CategoryId of 1. You also gave an alias to the
FavCategory table of the inner query. The alias is then used in the inner query’s WHERE clause and in the
third step when another query is nested inside the inner query from Step 2.
Step 3 sees an inner, inner query added to the previous subquery’s WHERE clause. Its role is to look for
members who already have category 9 as one of their favorites. It does this by checking that the innermost
query doesn’t return rows where the CategoryId is 9. The inner subquery, with its FavCategory
table with an alias of FC1, is linked to the innermost query where the FavCategory table has been given
the alias FC2. They are linked by MemberId, because you want to make sure when checking for a member’s
current favorites that it’s the same member. The second condition in the innermost query’s WHERE
clause checks to see whether the favorite category is one with an ID of 9.
In the final step, the outer query is used to provide the data for an INSERT INTO statement that adds
favorite category 9 as a member’s favorite.
Using Subqueries with the UPDATE Statement
Queries within Queries
As with INSERT INTO, you can use subqueries to supply values for updating or to determine the WHERE
clause condition in the same way that subqueries can be used with regular queries’ WHERE clauses. An
example makes this clear. Imagine that the film club chairperson has decided to boost profits by selling
DVDs. To maximize profits, she wants films that are rated higher than 3 and that appear in four or more
members’ favorite categories to have their prices hiked up to that of the highest price in the database.
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