The relationship rule of entities

the relationship rule of entities

The following table provides a summary of the guidelines for converting components of an entity-relationship. The entity relationship (ER) data model has existed for over 35 years. .. a project can have many employees working on it, depending on the business rules. 5. E/R Diagrams. ◇In an entity-relationship diagram: ◇Entity set Beers has two attributes, name and . Rule: if entity e is represented in a subclass, then e is .

But they exist only if they possess attributes equally.

the relationship rule of entities

Once decomposed the relationship, in the Physical Model, they will have the primary key of your super entity as a foreign key and also as a primary key. Any questions, please comment and I will answer.

EDIT The reaanb user raised an interesting question. I think it is valid to add content as the answer itself.

Use an entity or relationship in a rule

You the author of the question can even remove some doubts about what I said. When you decomposes or 1-N relations that has attributes, while maintaining the attributes in the relationship according to what has been mentioned in the commentsyou will have to create another table that need to at least have a foreign key to one of the sides of the relationship.

In terms of functionality, keep attributes on the N side can save you the creation of this table in case, you don't have to create this table. One less table within the final model is one less join operation, which can be costly if you need to do many times. In terms of readability, 1-N relationships represent unilateral relations.

Something that, in my view, always creates a dependency on a single side, not on both. Because of this, changes to the N side are simply irrelevant to the 1 side. In programming, development of programs and systems, at least, it follows the concept of decreasing the coupling between the parties, Which in this case, is a good thing even for the development of databases.

The only situation where this can complicate is for relations. In this type of occasion, we have to consider two things: Who, in the future, can change its cardinality to N? But it is rule that in every way we need to define a foreign key, right? We need to create a relationship after all.

For so cases, it is the discretion of the Data Modeler. So we generally put the foreign key on the side that can eventually become N. But this is not mandatory. Again, you must have discretion. Now, and when relations have attributes? That's a lot, if not impossible I kindly ask an example of context if anyone disagree with a plausible explanation of a real case, and if it exists, be sure that I will take back what I said.

  • Use an entity or relationship in a rule
  • Write rules that infer relationships and entities

If it happens it's good to review your diagram. For what reason these attributes are there? Are they relevant to both sides? Attributes in relationships are great indications that we have N-N or 1-N or N-1 relationships happening, or that you are putting attributes in the wrong place.

In this case simply just follow the aforementioned rules here.

Write rules that infer relationships and entities

How about a quick example to show practice about relationships? Let's imagine the following situation context: Imagine that in our company we have office rooms. Each office and employee has a proper name.

Imagine now that an office has only one employee, and an employee works in one office. This would give us the following Conceptual Model: So far so good, right? Now let's add one more detail: Each employee in every office must work about X hours a day.

Where will you add this information? Do not see the answer yet. Stop to think a little. Stop to think, really. Take a look at the context.

the relationship rule of entities

This information is relevant to whom? Let us develop a line of thought together Let's suppose we were to add a multi-valued attribute named "hours of work", right? Ideally, we transform this property into one entity directly, but for the sake of understanding, we will leave it as an attribute. This information needs to be persisted, so Where should we put this? If we use the relationship, then we are saying that when we do the decomposition of the Conceptual Model, the relationship will become a new table.

This table will have at least three fields: The last field will represent a foreign key to a new table that arose from the decomposition of a multi-valued attribute.

This relationship can accurately show us who, in which office, worked in which days for how long. But here are the caveats: You created one more table, which means one more join in queries. You may end up giving a wrong workload to the relationship established between the two entities if you get confused. You need to know in addition to the employee, the office as well.

Consider the following rule: A family may board the plane first if their child is under 8 years of age We know that families can have more than one child, however, this rule does not specify whether one or all of the family's children must be under 8 years of age in order for the family to board the plane first. If the family had 2 children, one aged 4 and one aged 16, how would you decide?

The rule would be clearer if written in such a way that the reader can tell whether the rule applies to one or all children.

Resolve m:n relationships

A family may board the plane first if they have at least one child under 8 years of age We use quantifiers, which are a type of Oracle Policy Modeling syntax, to write these kinds of rules. Quantifiers are operators which access data across the instances of an entity. The two quantifiers we use are: For example, "All of the apples are red". For example, "At least one of the bananas is yellow".

Check that a condition returns true for every instance of an entity The universal quantifier must be used when you refer from one entity to another entity in a "to-many" relationship, AND you need to determine whether all members of the target entity group need to satisfy the rule. This quantifier works in much the same way across entities as the 'and' operator does across attributes. This means that the rule using the universal quantifier will only evaluate to true when the condition is true for all instances of an entity.

In other words, the conclusion will evaluate to false if its condition is false for one of the targets of the relationship provided. There are two types of entity function that are used as universal quantifiers: This section describes the use of the For All function which is used where there is only one condition ie the rule only refers to one relationship. The use of the For All Scope function, where there are one or more conditions eg when you want to reason across several different relationships in the one ruleis more advanced and is covered in Extend the For, For All and Exists functions.

As mentioned above, the For All function is used where there is only one condition. For example, you could have the following rule where 'the family' is an entity the source entity'the child' is an entity the target entityand 'the family's children' is the relationship between the entities the relationship text.

Note that if there are zero instances of the entity, then the rule using the For All operator will evaluate to true. Check that a condition returns true for at least one instance of an entity The existential quantifier must be used when you refer from one entity to another entity in a "to-many" relationship, AND you need to determine whether any members of the target entity group need to satisfy the rule.

This quantifier works in much the same way across entities as the 'or' operator does across attributes.

Enhanced Entity-Relationship Model Part 2: Generalization vs. Specialization and EER Constraints

This means that only one instantiation of the entity must be true for the attribute using the operator to be true.