Detailed explanation of the foreign key constraint inspection method of closing child tables in MySQL (picture)

The following editor will bring you an article on how to check the foreign key constraints of MySQL closed subtables. The editor thinks it is quite good, so I will share it with you now and give it as a reference for everyone. Let’s follow the editor and take a look.

Preparation:

Define a teacher table and a student table; reference the teacher table in the student table ID

create table teachers(teacherID int not null auto_increment primary key,teacherName varchar(8));
create table students(studentID int not null auto_increment primary key,teacherID int not null,studentName varchar(8),
constraint fk_students_teacherID foreign key (teacherId) references teachers(teacherId) on delete no action on update cascade);

Step one:

Insert a teacher

insert into teachers(teacherName) values('NameA');

##Insert a student:

insert into students(studentName,teacherID) values('NameB',100);--可以知道没有这个教师号、所以插入会出错。

But is there a way to insert an unreasonable piece of data? There are still ways

Step 2:

set foreign_key_checks = 0; That’s it.

insert into students(studentName,teacherID) values('NameB',100)；

Step 3:

Set back to the default value and maintain foreign key constraints Prosecutor.

set foreign_key_checks =1;

Summary:

This essay is very messy. The main point I want to say is set foreign_key_checks =0; Foreign key constraints are of no use. At this time, inserts into the subtable that violate foreign key constraints can be performed.

Don’t use this unless absolutely necessary.

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