Relational database design theory
Functional Dependencies
Note A-> B denotes A decision function B , it can be said B A depends on function .
If {A1, A2, ...., An } is a set of one or more attributes of the relationship, the set function determines all other attributes and is the smallest , then the set is called the key code .
For A-> B, if A can find a proper subset of the set A ', is the A' -> B, then A-> B is partially dependent on the function , or is totally dependent function .
For A-> B, B-> C, then A-> C is a transfer function dependency .
abnormal
The following is a function of course is dependent on the relationship between students Sno, Cname-> Sname, sdept, Mname. Grade, key code is {Sno, Cname}. That is, after determining the curriculum and students, will be able to determine other information.
| Sno | come off | Sdept | Game | Cname | Grade |
|---|---|---|---|---|---|
| 1 | Students -1 | School -1 | Dean -1 | Course -1 | 90 |
| 2 | Students -2 | College -2 | Dean -2 | Courses -2 | 80 |
| 2 | Students -2 | College -2 | Dean -2 | Course -1 | 100 |
| 3 | Students -3 | College -2 | Dean -2 | Courses -2 | 95 |
Does not meet the relationship paradigm, it will have a lot of exceptions, mainly in the following four anomalies:
- Redundant data: for example, there were two students -2.
- Exception Review: modifying information in a record, but the other record has the same information has not been modified.
- To delete an exception: delete a message, it will also lose other information. Such as deleting a course-1, need to remove the first and third lines, the information-1 students will be lost.
- Insert an exception: for example, you want to insert the information a student, if the student has not elective, then it can not be inserted.
Paradigm
Paradigm theory in order to solve the above-mentioned four kinds of anomalies.
High-level paradigm relies on low-level paradigm , 1NF is the lowest level of paradigm.
1. The first paradigm (1NF)
Property can not be separated .
2. The second paradigm (2NF)
Each non-primary property entirely dependent on the key code .
It can be met by decomposition.
Decomposition ago
| Sno | come off | Sdept | Game | Cname | Grade |
|---|---|---|---|---|---|
| 1 | Students -1 | School -1 | Dean -1 | Course -1 | 90 |
| 2 | Students -2 | College -2 | Dean -2 | Courses -2 | 80 |
| 2 | Students -2 | College -2 | Dean -2 | Course -1 | 100 |
| 3 | Students -3 | College -2 | Dean -2 | Courses -2 | 95 |
Students course the above relationship, {Sno, Cname} key codes, the following functional dependency:
Sno-> detachable Sdept
Sdept->Mname
Sno,Cname->Grade
Grade entirely dependent on the key code, it has no data redundancy, each course each student has a specific result.
Sname, Sdept and Mname are partly dependent on the key code, when a student enrolled more than door class, these data will appear several times, causing a large amount of redundant data.
After decomposition
Relate to
| Sno | come off | Sdept | Game |
|---|---|---|---|
| 1 | Students -1 | School -1 | Dean -1 |
| 2 | Students -2 | College -2 | Dean -2 |
| 3 | Students -3 | College -2 | Dean -2 |
The following functional dependencies:
Sno-> detachable Sdept
Sdept->Mname
Relations -2
| Sno | Cname | Grade |
|---|---|---|
| 1 | Course -1 | 90 |
| 2 | Courses -2 | 80 |
| 2 | Course -1 | 100 |
| 3 | Courses -2 | 95 |
The following functional dependencies:
Sno,Cname->Grade
3. Third Normal Form (3NF)
Non-primary property does not depend on the transfer function of the key code .
-1 above relationship following transfer function dependencies:
Sno->Sdept->Mname
You can make the following decomposition:
Relations -11
| Sno | come off | Sdept |
|---|---|---|
| 1 | Students -1 | School -1 |
| 2 | Students -2 | College -2 |
| 3 | Students -3 | College -2 |
Relations -12
| Sdept | Game |
|---|---|
| School -1 | Dean -1 |
| College -2 | Dean -2 |
4.BCNF paradigm
3NF on the basis of, eliminating the properties of the main part of the function key code dependent and dependent transfer function