Database Reference
In-Depth Information
7.6 Summary and Concluding Remarks
Let us summarize what we have covered in this chapter:
•
The relational algebra is said to be prescriptive — you specify
precisely how an activity is to be carried out.
•
The eight basic operations of relational algebra are UNION,
MINUS, RESTRICT, PRODUCT, PROJECT, JOIN, INTERSECT and
DIVIDE BY. Each operation follows a specific syntax.
•
In addition to the basic operations, other relational operations
include ALIASES, RENAME, EXTEND and GROUP BY.
The next chapter discusses relational calculus as an equivalent alternative to
relational algebra.
7.7 Review Questions
1.
Why is relational algebra important?
2.
Briefly describe the basic operations of relational algebra.
3.
Using the college database described in this chapter, practice
writing relational algebra statements that will yield certain
desirable results.
4.
The following is an abridged specification of tables
comprising a music database:
Write relational algebra statements to realize the following:
a.
Registered musicians from CUB or CAN (where “CUB”
and “CAN” are abbreviated codes for Cuba and Canada
respectively).
b.
Give the ENO of every ensemble that includes a VIOLIN
or GUITAR player.
c.
Give the ENO of every ensemble that includes a VIOLIN
player but not a GUITAR player.
