Cardinality (data modeling) - Wikipedia
They are weak entity, multi valued attribute, derived attribute, weak relationship, and recursive relationship. Cardinality and ordinality are two. An Entity Relationship Diagram (ERD) is a visual representation of While cardinality specifies the occurrences of a relationship, ordinality. ER Diagram Representation - Learn DBMS in simple and easy steps starting from Cardinality is the number of instance of an entity from a relation that can be.
These are the attributes of the entity. Given the id of a desk, we would be able to seek out one specific desk, as no two would have the same id number.
It is shown by a line which normally connects two entities together. No cardinality or modality has been assigned yet. This is a 1: Each side of the relationship has a cardinality of 1.
The modality on each side is also 1.
ER Diagram Representation
A student must fill at least 1 seat, and 1 seat must be filled by at least one student. Although it may seem possible to have an empty seat, in which case the modality would be 0, the business rules we have defined determine that for the purposes of our database — we do not want to be able to store information about empty seats.
One instructor can teach many courses, but one course can only be taught by one instuctor. The modality is one on both ends of the relationship.
ER Diagram Representation
According to the business rules we have defined — an instructor must teach at least 1 course, and a course must be taught by 1 instructor. This is a M: One student can take many courses, and one course can be taken by many students.
For example, consider a database designed to keep track of hospital records. Such a database could have many tables like: There is a many-to-many relationship between the records in the doctor table and records in the patient table because doctors have many patients, and a patient could have several doctors; There is a one-to-many relationship between the department table and the doctor table because each doctor may work for only one department, but one department could have many doctors.
A "one-to-one" relationship is mostly used to split a table in two in order to provide information concisely and make it more understandable. In the hospital example, such a relationship could be used to keep apart doctors' own unique professional information from administrative details.
In data modelingcollections of data elements are grouped into "data tables" which contain groups of data field names called "database attributes". Tables are linked by "key fields".
A "primary key" assigns a field to its "special order table". For example, the "Doctor Last Name" field might be assigned as a primary key of the Doctor table with all people having same last name organized alphabetically according to the first three letters of their first name.
A table can also have a foreign key which indicates that field is linked to the primary key of another table. A complex data model can involve hundreds of related tables.