6.2 normalization 6.2.5 third normal form (3NF)

6.2 Normalization

 2NF What else is the problem ？
      The projection decomposition method is used , hold S-L-C Decompose into two relational patterns ：SC and S-L, Eliminated S-L-C Partial functional dependence of non primary attribute on code
      General , If so 1NF The relational schema is decomposed by projection , Eliminate partial functional dependence of non primary attributes on codes , Break down into several 2NF Relationship model of
      It can reduce the original 1NF Insert exception in relation pattern 、 Delete exception 、 Data redundancy marking 、 Modify complex problems
      But it cannot be completely eliminated Various exceptions and data redundancy in relational patterns

     2NF Relationship model S-L(Sno, Sdept, Sloc) in
Function dependency ：Sloc The transfer function depends on Sno, namely S-L Since the transfer function of non primary attribute pair code exists in Sno - Pass on -> Sloc

S-L Relationship problems ：
    （1） Insertion exception
     If one is for various reasons （ For example, it has just been established ）, Currently, there are no students in school , We will Unable to send the information of this department , Store in database .
    （2） Delete exception
     If all the students in a department graduate , While deleting the student information of the Department , Put the information of this department （Sdept、Sloc） Also lost .
    （3） Large data redundancy
     Students in every department live in the same place , The information about the Department's residence is repeated , The number of repetitions and the number of students in the Department identical
    （4） The modification is complicated
     When the school adjusts students' accommodation , Because the information about the residence of each department is repeatedly stored , When modifying All students in the Department must be updated at the same time Sloc Property value
  reason ：
    S-L in Sloc The transfer function depends on On Sno, That is, non primary attribute == Transfer function by code

  resolvent ：
The projection decomposition method is used , hold S-L Decompose into two relational patterns , To eliminate transfer function dependencies ：
    S-D(Sno, Sdept)
    D-L(Sdept, Sloc)
    S-D The code of is Sno,D-L The code of is Sdept
In the decomposed relational schema There are no non primary attribute pairs Part of the function depends on , There are no non primary attribute pairs The transfer function depends on , It further solves the above four problems
  The abnormal situation has been improved ：
（1）D-L The information of the system can be inserted into the relationship , Even if there are no students in school
（2） All the students in a department have graduated , Just delete S-D The corresponding tuple in the relationship ,D-L Information about the Department in the relationship still exists .
（3） Information about the Department's residence is just D-L In relationship Store once
（4） When the school adjusts the residence of students in a department , Only need to modify D-L In relationship One Of a tuple Sloc Property value

 3NF The definition of
Definition 6.7 Relationship model R<U, F>∈1NF, if R There is no such code in X、 Attribute group Y And non primary properties Z（Y⊉Z）, Yes X → Y,Y → Z,Y ↛ X establish , Then the layer R<U, F> ∈ 3NF（ There is no delivery dependency ）

chestnuts ：
S-D（Sno, Sdept） ∈ 3NF
D-L（Sdept, Sloc） ∈ 3NF

  Divide again without a column , Second, there is no partial dependence , Three non transitive dependencies

 3NF Some properties of ：
      if R∈3NF, be R Of Every non primary attribute Neither partial function depends on candidate code nor transfer function depends on candidate code
      If R∈3NF, be R∈2NF
      Using the projection decomposition method 2NF The relationship is decomposed into multiple 3NF The relationship between , Can be in a certain extent solve primary 2NF Insert exception in relation 、 Delete exception 、 Large data redundancy 、 Modify complex problems
      Will a 2NF The relationship is decomposed into multiple 3NF After the relationship , It can't completely eliminate Various exceptions and data redundancy in relational patterns