To begin with, the relational algebra is a
formal language. Today’s video provides two operators that are selection
operator and project operator. The selection operator is used to pick certain
rows and sigma represents the selection operator. The professor gives some
examples to illustrate the selection operator. For example, the question is
that students with a GPA greater than 3.7. The answer is that σ GPA > 3.7
Student. She also shows a relatively difficult question for us such as Students
with GPA > 3.7 and HS < 1000 something like that. The answer to this
question is that σ GPA > 3.7 ^ HS < 1000 Student. In this question,
symbol ^ is used to represent the word And. What’s more, she teaches the
project operator for us. The project operator is used to pick certain columns
and Pi π represents
the project operator. The professor gives some examples to show the function of
this operator as well. For instance, the question is the ID and decision of all
applications. The answer is that π Student
ID, decision Apply. It will also form a new table to show the result. Finally,
the prof shows how to pick both rows and columns. Her question is that ID and
name of students with GPA > 3.7. The answer to this question is that π student ID, student Name (σ GPA > 3.7 Student).
These operators are the basic knowledge for students to learn the new part of
the relational algebra. Professor Widom shows some relatively high-level
operators such as cross-product, natural join, and Union operator something
like that. The natural join should be a good example for us to explain it. For
one, it will enforce equality on all attributes with the same name. For
another, it will eliminate one copy of duplicate attributes. The natural join
is an easy method than the cross-product because it omits some unnecessary
steps. By watching today’s video, the professor shows how to use different
operators to figure out the different questions and also shows the function of
these operators.