Beginning SQL

Appendix A
382
You should use the following SQL if you’re using MS Access:
SELECT C1.Category, MD1.FirstName, MD1.LastName, MD2.FirstName, MD2.LastName
FROM (((FavCategory AS FC1 INNER JOIN FavCategory AS FC2
ON FC1.CategoryId = FC2.CategoryId)
INNER JOIN Category AS C1 ON C1.CategoryId = FC1.CategoryId)
INNER JOIN MemberDetails MD1 ON MD1.MemberId = FC1.MemberId)
INNER JOIN MemberDetails MD2 ON MD2.MemberId = FC2.MemberId
WHERE FC1.MemberId < FC2.MemberId;
It might look horrendously long, but broken down into its parts it’s not that bad. The very first step is to
pair up people with the same favorite category. To get pairs of people in the same row, you need to use a
self-join of the FavCategory table:
SELECT FC1.CategoryId, FC1.MemberId, FC2.MemberId
FROM FavCategory FC1 INNER JOIN FavCategory FC2
ON FC1.CategoryId = FC2.CategoryId
WHERE FC1.MemberId < FC2.MemberId;
The WHERE clause is there to prevent the same person from being listed as a pair of themselves on the
same row. This gives you the CategoryId and MemberIds involved, but the question requires the category
name and the first and last names of the people. First, get the category title from the Category table
by joining it to the current results:
SELECT C1.Category, FC1.MemberId, FC2.MemberId
FROM FavCategory FC1 INNER JOIN FavCategory FC2
ON FC1.CategoryId = FC2.CategoryId
INNER JOIN Category C1 ON C1.CategoryId = FC1.CategoryId
WHERE FC1.MemberId < FC2.MemberId;
Now you need to get the first and last names. For this, you need to join the MemberDetails table to the
current results. However, on the same row are two different MemberIds: for FC1 and for FC2. You can’t
just join MemberDetails once; you need two copies of the table joined. The first copy provides the first
and last names for FC1’s MemberId, and the second copy provides the lookup for the first and last
names from FC2’s MemberId:
SELECT C1.Category, MD1.FirstName, MD1.LastName, MD2.FirstName, MD2.LastName
FROM FavCategory FC1 INNER JOIN FavCategory FC2
ON FC1.CategoryId = FC2.CategoryId
INNER JOIN Category C1 ON C1.CategoryId = FC1.CategoryId
INNER JOIN MemberDetails MD1 ON MD1.MemberId = FC1.MemberId
INNER JOIN MemberDetails MD2 ON MD2.MemberId = FC2.MemberId
WHERE FC1.MemberId < FC2.MemberId;