第67页 3.3.2 Queries on Multiple Relations

In contrast,
the attributes name and building appear in only one of the relations, and therefore
do not need to be prefixed by the relation name.
• The select clause is used to list the attributes desired in the result of a query.
• The from clause is a list of the relations to be accessed in the evaluation of
the query.
• The where clause is a predicate involving attributes of the relation in the
from clause.

A typical SQL query has the form
select A1, A2, . . . , An
from r1, r2, . . . , rm
where P;

Each Ai represents an attribute, and each ri a relation. P is a predicate. If the where
clause is omitted, the predicate P is true.