This set entered on 5-12-2014
5.1.4.3
- p 312. Now that you have the list of entities and their relationships, you’re ready to assemble the MASTER PLAN, the;
ENTITY RELATIONSHIP DIAGRAM
- What is an ENTITY RELATIONSHIP DIAGRAM?
it is a picture that shows all the entities in the database AND THE RELATIONSHIPS BETWEEN THEM. EACH ENTITY APPEARS ONLY ONCE IN THE ENTITY RELATIONSHIP DIAGRAM.
- What is the purpose of the ENTITY RELATIONSHIP DIAGRAM?
It has TWO purposes:
a. To help find relationships you missed, or that don’t belong
b. To serve as a ROADMAP for the database.
- What is the GOAL when creating the ENTITY RELATIONSHIP DIAGRAM?
a. To have each ENTITY in the picture just once
b. to have all the lines necessary to explain the various relationships.
- How do you start the ENTITY RELATIONSHIP DIAGRAM?
You start by drawing one box for EACH ENTITY you’ve identified, THEN drawing the lines that represent each of the relationships.
- If two entities are connected by a PATH ALONG RELATIONSHIP LINES, -EVEN THROUGH OTHER ENTITIES, then they are:
RELATED as far as FileMaker is concerned. They don’t need a direct line between them.
- What is an IMPLIED RELATIONSHIP?
When two tables are INDIRECTLY CONNECTED though another table.
- What are CROW’S FEET?
It denotes the “MANY” side of a relationship, and is drawn as something resembling a bird’s foot (Crow’s Foot) on the MANY side. The “ONE” side of the relationship only has a straight line connection.
- When thinking about IMPLIED RELATIONSHIPS, pay attention to the CROW’S FEET. If, when moving from one entity to another along the relationship lines, you ever go through a TO MANY RELATIONSHIP, then:
the LARGER IMPLIED RELATIONSHIP is itself TO MANY
- If a CUSTOMER has more then one JOB, and each JOB has EXPENSES, then clearly:
A CUSTOMER CAN HAVE MORE THAN ONE EXPENSE.
- What is REDUNDANCY in a RELATIONSHIP?
They are LINES that show DIRECT RELATIONSHIPS when the RELATIONSHIP is already IMPLIED by OTHER ENTITIES and RELATIONSHIPS. You SHOULD ALWAYS GET RID OF REDUNDANCY.
- What is a RAT’S NEST?
Too many lines that don’t need to be there, aka REDUNDANCY IN THE ENTITY RELATIONSHIP DIAGRAM.
