MySQL study notes three

relation

• Create score table scores, the structure is as follows
  • id
  • student
  • suject
  • score
• Thinking: What information should be stored in the student column?
• Answer: The data of the student column is not newly created here, but should be referenced from the student table, and the relationship is also a piece of data; according to the requirements of the paradigm, the student's number should be stored, not the student's name and other information
• Similarly, the chart of accounts is also a relational column, referencing the data in the chart of accounts

 

• The statement to create the table is as follows

create table scores(
id int primary key auto_increment,
study int ,
subid int,
score decimal(5,2)
);

foreign key

• Thinking: How to ensure the validity of relational column data? Is any integer ok?
• Answer: It must be the data existing in the id column in the student table, and the validity of the data can be verified through foreign key constraints
• Add foreign key constraint for stuid

alter table scores add constraint stu_sco foreign key(stuid) references students(id);

• When inserting or modifying data at this time, if the value of stuid does not exist in the students table, an error will be reported
• Constraints can be created directly when creating a table

create table scores(
id int primary key auto_increment,
study int ,
subid int,
score decimal(5,2),
foreign key(stuid) references students(id),
foreign key(subid) references subjects(id)
);

 

Cascading operations on foreign keys

• When deleting the data of the students table, if the id value already exists in the scores, an exception will be thrown
• It is recommended to use logical delete, which can also solve this problem
• You can specify cascading operations when you create a table, or you can modify the cascading operations of foreign keys after you create the table
• grammar

alter table scores add constraint stu_sco foreign key(stuid) references students(id) on delete cascade;

 

• Types of cascading operations include:
  • restrict: default value, throw exception
  • cascade (cascading): if the record of the main table is deleted, the associated records from the table will be deleted
  • set null: set the foreign key to null
  • no action: do nothing