Basic operation mysql database (VI)

 

Foreign key constraint

Create a foreign key

       Queries are speaking in front of a table form, but the project between the tables and the tables are related, such as student table that we created, they may be in different classes, different classes have different class teacher, about the relationship between them is this: each class will correspond to more students, and each student corresponds to only one class. So create a teacher's table, this table should be the main table, the students of that table is the child table, you need to add class information:

CREATE TABLE Classhost(

       id TINYINT PRIMARY KEY auto_increment,
       name VARCHAR (20),
       age INT ,
       is_marriged boolean 
);

INSERT INTO Classhost (name,age,is_marriged) VALUES ("丽丽",22,0),
                                                       ("莉莉",24,0),
                                                       ("李丽",22,0),
                                                       ("李莉",20,0);
CREATE TABLE student(
        id INT PRIMARY KEY auto_increment,
       name VARCHAR (20),
       host_id TINYINT   
)ENGINE=INNODB;

INSERT INTO Student(name,host_id) VALUES ("pengfy",2),
                                            ("pyq",4),
                                            ("xiaojiang",1),
                                            ("pyq2",3),
                                            ("pengfy2",1),
                                            ("pyq3",3),
                                            ("lily",2);

     Created the two tables below:

 

     Now suppose Lily resigned, we try to delete name is Lily's teacher:

delete from classhost where name="丽丽";

     Really deleted, now take a look at the student table has not changed:

        Found xiaojaing and pengfy2 teacher id or 1, this is clearly inappropriate, a teacher is gone, student information should change, or assigned to other classes, or go together (especially liked the teacher), but we also try after deleting teacher, student information insert a table in the student inside, host_id set to 1, it is OK. In both cases the display is unreasonable, then the time will the introduction of the concept of foreign keys, and now the students add a foreign key table, we must first find a place associated with two tables, the table is that students and teachers inside host_id table inside the id is associated, that we look at new students table 2:

CREATE TABLE student2(
        id INT PRIMARY KEY auto_increment,
       name VARCHAR (20),
       host_id TINYINT,    --切记:作为外键一定要和关联主键的数据类型保持一致       
       FOREIGN KEY(host_id) REFERENCES classhost(id)
)ENGINE=INNODB;

INSERT INTO Student2(name,host_id) VALUES ("pengfy",2),
                                            ("pyq",4),
                                            ("xiaojiang",4),
                                            ("pyq2",3),
                                            ("pengfy2",4),
                                            ("pyq3",3),
                                            ("lily",2);

       这里新的学生信息也要改一下，把id为1的要改成其他的，不然会报错。现在再试一下，删掉id为2的班主任：

delete from classhost where id=2;

       报错了，不让删除了，因为外键约束了，报错信息如下：

 Cannot delete or update a parent row: a foreign key constraint fails (`test`.`student2`, CONSTRAINT `student2_ibfk_1` FOREIGN KEY (`host_id`) REFERENCES `classhost` (`id`))

      再试一下给id=1的老师加一个学生，看是否能添加成功：

insert into student2(name,host_id) values ('lucy',1);
报错信息：
Cannot add or update a child row: a foreign key constraint fails (`test`.`student2`, CONSTRAINT `student2_ibfk_1` FOREIGN KEY (`host_id`) REFERENCES `classhost` (`id`))

     同样报错了 ，看一下报错信息里面有个东西，CONSTRAINT `student2_ibfk_1`这个东西我没有写过，后面的FOREIGN KEY (`host_id`) REFERENCES `classhost` (`id`))才是我写的，那这个是什么呢?这个是外键名称，你不自定义的时候，会默认给你加上一个，所以给一个表增加一个外键完整的写法是这样的：

ALTER TABLE student  ADD CONSTRAINT anyname
                     FOREIGN KEY(host_id)
                     REFERENCES  classhost(id);

     试着给我们学生表1增加一个外键：

 Cannot add or update a child row: a foreign key constraint fails (`test`.`#sql-1110_2`, CONSTRAINT `anyname` FOREIGN KEY (`host_id`) REFERENCES `classhost` (`id`))

     也报错了，这是为什么呢？因为老师里面已经没有id=1的了，而学生表还存在，看来要修改一下学生表才行：

     第三个和第五个改成4好了，

update student set host_id=4 where id=3 or id =5;

     再试一下增加外键，看一下表结构：

      添加成功，那么能增加就能删除嘛，试一下删除这个外键：

 ALTER TABLE student DROP FOREIGN KEY anyname;

      查看是否删除成功：

     已经找不到外键了，删除成功喽。那现在的问题就是主表怎么正常删除数据，并且把子表的数据也一并删除。这就涉及下面要讲的INNODB支持的ON语句。

INNODB支持的ON语句

    外键约束对子表的含义:   如果在父表中找不到候选键,则不允许在子表上进行insert/update
    外键约束对父表的含义: 在父表上进行update/delete以更新或删除在子表中有一条或多条对应匹配行的候选键时,父表的行为取决于：在定义子表的外键时指定的on update/on delete子句

innodb支持的四种方式

1.cascade方式

      在父表上update/delete记录时，同步update/delete掉子表的匹配记录，外键的级联删除：如果父表中的记录被删除，则子表中对应的记录自动被删除，比如刚才在删除班主任的同事，也把学生删除，就应该在主键中加上一段ON DELETE CASCADE：

FOREIGN KEY (charger_id) REFERENCES ClassCharger(id)
                              ON DELETE CASCADE

      我们再给student加上这么一个外键：

ALTER TABLE student  ADD CONSTRAINT anyname
                     FOREIGN KEY(host_id)
                     REFERENCES  classhost(id)
                     ON DELETE CASCADE;

     同时我们也要把student2这张表删除，因为主表也和这个表绑定了，外键没改到时候删除主表还是会报错的，删掉之后，试一下删除classhost表里面的一个内容：

     成功了，没有报错啊，看看子表里面，原先我们的子表id=1和id=7都是host_id=2的：

      现在子表里面绑定了host_id=2的信息也都一起删除了，除了这种，还有以下三种情况。

2.set null方式

      在父表上update/delete记录时，将子表上匹配记录的列设为null，要注意子表的外键列不能为not null

FOREIGN KEY (charger_id) REFERENCES ClassCharger(id)
                              ON DELETE SET NULL

      有兴趣可以试一下这种方式，就不演示了。

3.Restrict方式

       拒绝对父表进行删除更新操作，这个很少用，相当于不让sql报错，了解一下就行了。

4.No action方式

      在mysql中同Restrict,如果子表中有匹配的记录,则不允许对父表对应候选键进行update/delete操作，这个也作一个了解就行。