Do a project, it is indispensable necessity database design! In the learning phase, basically single table. However, in the actual development process, one-to-many tables everywhere! Simple and tidy, one-to-many table to sort out how to design ideas:
There are three correspondence between database entities: one to one, one to many, many to many.
One relationship Example:
- A student archival material corresponds to a student, or each person has a unique ID number.
Many relationship example:
- A student belongs to only one class, but a class a number of students.
Many relationship example:
- Students can choose a multi-course, a course also a number of students.
First, the realization relationship
In the entity-relationship model, we know that there are three relationships: one to one, one to many, many to many . This is only a conceptual relationship, but in a real relational database, we have only foreign keys ,
These three relationships are not, then we'll talk about in a relational database management system, how to achieve these three relationships.
Many
Here to explain many, because this relationship is the simplest. And many-to-many is one thing, so I will not mention many-word. Is a concept many object A will correspond to a plurality of objects B,
From the angle B of view, an object B corresponds to only one object A. For example, classes and students is one to many relationship. A corresponding number of students in class, a student will only be for a class.
The reason for that is simple-to-many relationship, because the RDBMS [foreign keys] in fact represent many relationship. For many relationship, we only need to create foreign key "one" to "many" of the table ,
The "one" side of the table do not need to make any changes. For example, students in the class relations said before. Class table unchanged, increase student table as a foreign key Id class.
