RELATIONAL DATA MODEL FUNDAMENTALS - Database Design and Development: An Essential Guide for IT Professionals - page 253

DrID DrName Specialty DrPhone SuprDrID

45 Robert Martin Surgery General 609-924-9248 25

36 Elaine Hsu Urology 908-535-5228 25

13 Leroy Richardson Oncology 908-322-5566 25

DOCTOR Relation

10

14
23

Explanation of
INTERSECTION operation

45

37 25

36

13

39

DrID DrName Specialty DrPhone SuprDrID

10 Kim Maggio Family Practice 918-756-0831 25

14 Monika Isaac Neurology 609-238-3377 45

23 Marcus Kulper Internal Medicine 732-613-9313 36

37 Ira Goldman Psychiatry 609-246-0331 13

39 Lawrence Stark Radiology 918-613-1988 45

SUBORDDOCTOR Relation

10

14
23

Explanation of
DIFFERENCE operation

45

37 25

36

13

39

Figure 8-13 INTERSECTION and DIFFERENCE operations.

Relational algebra solution
DOCTOR :

= SUPVRDOCTOR « SUBDOCTOR

Note the syntax for the operation in relational algebra and the symbol indi-

cating the intersection operation. The intersection operation finds only those
entities that are common to both relations. Intersection operation in relational
algebra is similar to intersection function in mathematical set theory. Note the
solution shown in Figure 8-13.

«

Difference

Features and function

• Operates on two relations.
• Identifies the rows in one relation that are not present in the other relation.
• Both relations must be union-compatible; that is, they must have the same
columns.
• The result is a single relation containing rows and columns as indicated
below:
• Rows—only those rows that are in the first relation but not in the second
• Columns—same columns as in either of the original relations

Example

Refer to Figure 8-11 for the discussion of this operation.

Data request List all the attributes of all doctors who report to some other
doctors but have no one reporting to them. These are the doctors who are sub-