RELATIONAL DATA MODEL FUNDAMENTALS - Database Design and Development: An Essential Guide for IT Professionals - page 254

ordinates without any supervisory responsibilities. Call the result relation
SUBORDDOCTOR.

Relational algebra solution
SUBORDDOCTOR :

= SUBDOCTOR — SUPVRDOCTOR

Note the syntax for the operation in relational algebra and the symbol — indi-
cating the difference operation. The difference operation finds only those enti-
ties in the SUBDOCTOR relation but not present in the SUPVRDOCTOR
relation. These are subordinates that do not have any supervisory responsi-
bilities at all. Note the solution shown in Figure 8-13.
Also note that the result will not be same if you reverse the order of the two
relations in the difference operation.
(A—B) is not the same as (B—A). (SUPVRDOCTOR — SUBDOCTOR) rep-
resents the doctors who only have supervisory responsibilities and do not
report to any other doctor at all.

Product

Features and function

• Operates on two relations.
• Produces the scalar product between the two relations.
• By itself, the product operation has no meaning in business functions. But the
product operation is an essential interim operation for the more important join
operation.
• The result is a single relation containing rows and columns as indicated below:
• Rows—formed by concatenation each of row of one relation to each row of
the other relation
• Columns—all the columns of both relations put together

Example

Refer to Figure 8-14 for the discussion of this operation. Note the two relations
A and B.

Data request Find the product relation C between two relations A and B.
Relational algebra solution

C : = A * B

Note the syntax for the operation in relational algebra and the symbol * indi-
cating the product. Observe how the rows for relation C are formed from the
rows of A and B.
Let us generalize the product operation.
If relation A has m columns and p rows, and relation B has n columns and q
rows, then the result relation C will have (m+n) columns and (p * q)

rows.