Beginning SQL

Chapter 3
SQL Is Set-Based
102
You might remember from your high school math days the concept of the set, which is simply a collection,
in no particular order, of items of the same type. SQL is set-based, the sets being sets of records. As
with sets in math, there is no particular order to SQL data sets unless you specify the order, using the
ORDER BY clause, for example. With the more straightforward queries (like the ones earlier in the chapter),
considering the set-based nature of SQL queries isn’t really necessary. However, with trickier
queries, especially those involving more than one table, thinking in terms of sets is helpful.
Taking the example from the previous section, examine how the first step looks as a set. The first step’s
SQL is as follows:
SELECT FavCategory.CategoryId, FavCategory.MemberId
FROM FavCategory;
Represented as a set diagram, the first step looks like Figure 3-2.
Figure 3-2
The set simply contains all the records and columns from the FavCategory table.
The next step joins the Category and FavCategory tables with this SQL:
SELECT Category.Category, FavCategory.MemberId
FROM FavCategory INNER JOIN Category
ON FavCategory.CategoryId = Category.CategoryId;
Figure 3-3 shows a set diagram of the SQL.
SELECT FROM FavCategory
CategoryId
1
1
1
2
2
3
2
6
1
MemberId
3
5
10
1
3
3
14
14
3