[introduction to database system (Wang Shan)] Chapter 5 - database integrity

1、 The integrity of database refers to the correctness and compatibility of data .

2、 Integrity control mechanism ：

       （1） Provide a mechanism for defining integrity month conditions
        （2） Provide methods for integrity checking
       （3） Default treatment

3、 Entity integrity definition ：

       PRIMARY KEY Definition , Table level integrity constraints or column level integrity constraints .
        Entity integrity ： The primary property cannot be empty .

 example ：CREATE TABLE Student
        (Sno       CHAR(5)  PRIMARY KEY,   /* Define the main code at the column level */
         Sname  CHAR(20)  NOT NULL,          
         Ssex      CHAR(1) ,
         Sage      INT,
         Sdept    CHAR(15));
       CREATE TABLE Student
        (Sno       CHAR(5) , 
         Sname  CHAR(20)  NOT NULL,          
         Ssex      CHAR(1) ,
         Sage      INT,
         Sdept    CHAR(15),
         PRIMARY KEY （Sno）            /* Define the main code at the table level */
         );
        CREATE TABLE SC
        (Sno CHAR(9) NOT NULL,
         Cno CHAR(4) NOT NULL,
         Grade SMALLINT,
         PRIMARY KEY （Sno,Cno）        /* A set of attributes is the main code , The main code can only be defined at the table level */
         ）;

4、 Entity integrity check and default handling

        According to the entity integrity rules , The system checks automatically :
        1） Whether the primary code value is unique .
        2） Whether the main code attributes are empty .

5、 Refer to integrity definition ： Create table time ,FOREIGN KEY Definition .

Referential integrity ： The outer code value is null or equal to the value of a Yuanzu of the corresponding main code .

     example ： CREATE TABLE SC(
            Sno CHAR(5) ,
            Cno CHAR(3) , 
            Grade   int,
            Primary key (Sno, Cno)
            foreign key (Sno) references Student(Sno),
            foreign key (Cno) references Course(Cno)
  );

6、 Several issues to consider in realizing referential integrity :

        1） Whether the outer code can accept null value
        2） The problem of deleting tuples in the referenced relationship
        3） Problems in inserting tuples into reference relationships
        4） Modify the master code in the relationship

1） Whether the outer code can accept null value

        When implementing referential integrity , The system should provide a mechanism for defining foreign codes , A mechanism should also be provided to define whether the outer code column allows null values .

  example 1：
	 Student （ Student number , full name , Gender , Major number , Age ）
	 major （ Major number , Professional name ）
	 The major number column in student relations can be null .
   example 2：
	 Student （ Student number , full name , Gender , Major number , Age ）
	 Course （ Course no. , Course name , credits ）
	 Elective （ Student number  , Course no. , achievement ）
	 The student number column in the elective relationship cannot be null .

2） The problem of deleting tuples in the referenced relationship

        In a general way , When deleting a tuple of the referenced relationship , There are several tuples in the reference relationship , Its outer code value is the same as the main code value of the deleted tuple of the referenced relationship , Can there be at this time Three Different Strategy ：
       (1) cascading deletion （CASCADES）
       (2) Restricted deletion （RESTRICTED）
        Only if the outer code value of no tuple in the reference relationship is the same as the main code value of the tuple to be deleted in the referenced relationship , The system only performs the deletion operation , Otherwise, reject the deletion .
        (3) Set null to delete （NULLIFIES）
        Delete the tuple of the referenced relationship , And set the outer code value of the corresponding tuple in the reference relationship to null .
        These three methods , Which one is right , It depends on the semantics of the application environment

3） Problems in inserting tuples into reference relationships

        In a general way , When a tuple is inserted into a reference relationship , There is no corresponding tuple in the referenced relationship , Its main code value is the same as the outer code value of the tuple inserted by the reference relationship , Here are some strategies ：
        （1） Restricted insertion
        Only if there are corresponding tuples in the referenced relationship , Its main code value is the same as the outer code value of the tuple inserted by the reference relationship , The system performs the insertion operation , Otherwise, reject this operation .
       （2） Recursive insert
        First, insert the corresponding tuple into the referenced relationship , Its main code value is equal to the outer code value of the tuple inserted by the reference relationship , Then insert tuples into the reference relationship .

4） Modify the master code in the relationship

       (1) It is not allowed to modify the master code
In some RDBMS in , Modifying the relationship master code is not allowed .
       (2) It is allowed to modify the master code
        In some RDBMS in , It is allowed to modify the relationship master code , However, the uniqueness and non emptiness of the main code must be guaranteed , Otherwise, the modification is rejected .
        If the referenced relationship is modified , Is similar to deleting , Yes Three strategies ：
        Cascade modification ： While modifying the primary code value of the referenced relationship , Modify the corresponding foreign code value in the reference relationship .
        Restricted modification ： Reject this modification . Only when the outer code value of no tuple in the reference relationship is equal to the main code value of a tuple in the reference relationship , Can be modified .
        Set null value to modify ： While modifying the primary code value of the referenced relationship , Set the corresponding foreign code value in the reference relationship to null .

 example ：     CREATE TABLE SC(
            Sno CHAR(5)  NOT NULL,
            Cno CHAR(3) NOT NULL, 
            Grade   SMALLINT,
            Primary key (Sno, Cno)
            foreign key (Sno) references Student(Sno),
                   ON DELETE CASCADE     Delete level company 
                   ON UPDATE CASCADE,    Company update 
            foreign key (Cno) references Course(Cno)
                   ON DELETE CASCADE
                   ON UPDATE CASCADE);

7、 User defined integrity ： Definition of constraints on tuples .

        Column value is not empty （NOT NULL） Column value uniqueness (UNIQUE) CHECK The phrase

   example ：  CREATE TABLE Student
        (Sno       CHAR(5) , 
         Sname  CHAR(20)  NOT NULL,          
         Ssex      CHAR(1) ,
         Sage      INT,
         Sdept    CHAR(15),
         PRIMARY KEY （Sno）
         CHECK （Ssex=‘ Woman ’ OR Sname NOT LIKE ‘Ms.%’）
         );

8、 Integrity constraint named clause

CREATE TABLE Student
    (Sno       NUMERIC（6）
                 CONSTRAINT C1 CHECK（Sno BETWEEN 90000 AND 99999）, 
     Sname  CHAR(20)  
                 CONSTRAINT C2 NOT NULL,          
     Ssex      CHAR(2) 
                  CONSTRAINT C3 CHECK（Ssex IN （’ male ‘,’ Woman ‘））,
     CONSTRAINT C4      PRIMARY KEY （Sno）
      );

  notes ：C1、C2、C3、C4 Is the constraint name     constraint Define the keyword for the constraint name 

        Modify the integrity restrictions in the table

 ALTER TABLE Student DROP CONSTRAINT C4;
   ALTER TABLE Student ADD CONSTRAINT C1 CHECK（Sno BETWEEN 900000 AND 999999）;