N-ary relationship types
o self study. ❑ ER Construct Notation Comparison Translation of a (Normal Form) ER Diagram to a RDB . Different cardinalities of binary relationship types. The goal of the E-R modeling process is to create an E-R diagram, which we can then . We did model it as an entity, but could we model it as a ternary relationship . The above Chen-style diagrams are characterized by separate symbols for Cardinality is represented by how the ends of these lines are decorated. When we design a database, we draw an entity relationship diagram (ERD). It helps us understand what kind of information we want to store.
N relationships Here we must create a table R of tuples including the key of S skthe key of T tkand any attributes of R; we can not push the data into either S or T.
The sk column of R should have a foreign key constraint referring to the key column of S, and the tk column of R should similarly have a foreign key constraint to the key column of T. Again, we would have an appropriate foreign key constraint back to the original table. Make this a foreign key in E1. Make this new column a foreign key in E2, referencing E1. For multivalued attributes of entity E, create a new relation R.
One column of R will be E. Joins arise in steps 2, 3, 4, 5, 6, and 7, for recovering the original relationships or attribute sets for 6, or entities for 2. In every case, the join field is a key of one relation and a foreign key in the other. Not all joins are about recovering relations from an ER diagram.
Also, I said earlier that entity T should not have an attribute that was another entity of type S; instead, we should create a relationship R between T and S. If S was at all a candidate for an attribute, each T would be related to at most one S and so this would have cardinality constraint TNRS.
Then, when we did the above conversion, in step four we would add S's key to T with a foreign key constraint referring to S. But suppose we did add S as an entity attribute to T. Then we would end up with the same situation: So in the end we get the same thing. As we see, a person can be in the relationship with another person, such as: This is definetly the most used relationship type.
Journalist writes an article. This example can be implemented very easily. In the diagram below, we represent our ternary relationship with an extra table, which can be modelled in Vertabelo very quickly. In other words, a group can have specific classess only at one classrom. Sometimes it is possible to replace a ternary or n-ary relationship by a collection of binary relationship connecting pairs of the original entities.
However, in many cases it is hard to replace ternary relationship with two or more binary relationships because some information could be lost.
Another ternary relationship presents a different situation — Teacher recommends a book for a class: If the system user will use the individual components, then model the system in that fashion. Sometimes an attribute can be calculated.
For such simple attributes, we usually do not store the attribute. Sometimes you can derive an attribute from attributes of several related entities. If the calculation of the value is computationally intensive, then you may elect to store the derived TotalAmount attribute.
database - ERM: Cardinality in ternary relationships - Stack Overflow
Entity Type - Entity Instance. An Entity Type is a collection of occurrences of entities that have common properties attributes. An Entity Instance is a single occurrence of an entity type, e. Multivalued attributes are often shown in a diagram by the use of a double circle as is done in the figure shown above.
Multivalued attributes may have multiple values for an individual occurrence of an entity. As an example, a customer may have multiple different telephone numbers CustomerPhone. A Candidate Key is an attribute or set of attributes that uniquely identify an instance of an entity, e. Some entities have more than one candidate key. Sometimes it is necessary to combine attributes to form a composite key out of two or more attributes, e. A primary key is denoted by underlining an attribute in an ER diagram as shown in the figure above CustomerNumber.
A primary key is usually a single attribute, but may be a composite key combination of two or more attributes. The term identifier is also used for primary key.
In the case of an ER modeling diagram, the term identifier is more technically correct; however, the use of the term primary key tends to dominate in industry. The primary key selection must follow certain criteria. Key should NOT change its value over the life of each entity instance.
Avoid intelligent-keys stored data as part of a key such as a code. For example, a key where the first two digits indicates a warehouse location would be a poor choice because the coding scheme could change over time. Substitute single-attribute surrogate keys for large composite keys.