DATABASE SYSTEMS COURSEWORK Essay Example | Topics and Well Written Essays - 1500 words

DATABASE SYSTEMS COURSEWORK - Essay Example The relationship type 'has_details' binds the student, invoice and room entities. As the invoice entity has details of both the student and the room and also its own properties needs to be linked with all the three to depict the relation. d. The relationship 'reside' links student and hall entities. As students reside in the hall in a particular room. This conceptual model depicts all the entities and their relations; it gives the overall picture and understanding of the system to a novice. This can also be called a logical model as it does not take into consideration any DBMS in mind and shows the logical association of the system. Next we are constructing the logical system which has the cardinality ratios. A cardinality ratio of a binary relation is the maximum number of instances an entity can participate in a given relationship type. Logical model The logical model is as follows: The difference here is that the entities here are associated with their relationships with the cardinality ratios respectively. The cardinality ratios are derived as follows: 1. Between student and staff (M:N). a. 1 student is responsible to 1 staff b. Many (M) students are responsible to 1 staff. c. 1 staff takes responsibility of 1 student d. Many (M) staff takes responsibility of many students. 2. Between student and hall (N:1). a. 1 student resides in 1 hall. b. Many (M) students reside in 1 hall. c. 1 hall accommodates 1 student d. Many (M) halls do not accommodate 1 student. 3. Between student, hall and invoice (1:1:1). a. 1 invoice has details of 1 student and his hall. b. 1 invoice does not have details of more than 1 student and their halls. c. Many invoices do not have details of many students and different halls. 4. Between student and room (1:1). a. 1 student... This conceptual model depicts all the entities and their relations; it gives the overall picture and understanding of the system to a novice. This can also be called a logical model as it does not take into consideration any DBMS in mind and shows the logical association of the system. Next we are constructing the logical system which has the cardinality ratios. A cardinality ratio of a binary relation is the maximum number of instances an entity can participate in a given relationship type. Special note: The attribute named 'lease' to the relationship named 'rent' has been shifted to the rent table as the cardinality ratio is 1:1 so any attribute of the relationship type must be shifted on either side. The arrows show the functional dependencies. The single arrow with no point head determines all the attributes of the entire entity. In the table 'Responsible_For' which was previously a relationship between student and staff had a cardinality ratio of M:N. The relationship type will form a separate table with the same name having the primary keys of both the aligned tables. a. Only the table 'Room' has an attribute called 'Room Type' which can have both 'single' and 'double' rooms in a particular hall.