Simple Record Database

(extracted from the paper of Transaction Logic Programming)

Suppose a university has a database of students, courses, and professors, the database includes the following four base relations:

takes(Stud,Crs,Sec), which records the students enrolled in each section of each coruse.
enrolled(Crs,N), which records the total number of students, N, enrolled in a course.
instructs(Prof,Crs,Sec), which records the professors who teach each section of each course.
load(Prof,N), which records each professor's course load, N, i.e., the total number of classes that he teaches.

Using the following transaction program can ensure database consistency when a student drops a course or a professor is relieved from teaching a course.

declare takes/3.
declare enrolled/2 key 1.
declare instructs/3.
declare load/2 key 1.
drop(Stud,Crs,Sec) <- destroy(takes(Stud,Crs,Sec) sc decr_enrolled(Crs).
relieve(Prof,Crs,Sec) <- destroy(instructs(Prf,Crs,Sec)) sc decr_load(Prof).
decr_encrolled(Crs) <- enrolled(Crs,N) sc N1 is N-1 sc set(enrolled(Crs,N1)).
decr_load(Prof) <- load(Prof,N) sc N1 is N-1 sc set(load(Prof,N1)).

When a student drops a course, the program decrements the enrollment total and when a professor is relieved from teaching a course, the professor's course load is decremented.

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