Lesson 4 Basic _ constraint

Lesson 4 Basic _ constraint

1. Summary

1. Concepts

Rules that act on fields in a table to restrict data stored in the table.

2. Purpose

Ensure that the data is correct, valid and complete.

3. Classification

constraintdescribeKeyword
Non-empty constraintRestrict the field to data that cannot be nullNOT NULL
Unique ConstraintEnsure that all data in this field is unique and not duplicatedUNIQUE
Primary Key ConstraintPrimary keys are the only representation of a row of data, requiring non-empty and uniquePRIMARY KEY
Default ConstraintsWhen saving data, if no value is set for the field, the default value is usedDEFAULT
Check constraints (after version 8.0.16)Ensure that a field meets a conditionCHECK
Foreign Key ConstraintsUsed to connect data between two tables to ensure data consistency and integrityFOREIGN KEY

Note: Constraints are used for fields in tables and can be added when creating/modifying tables.

2. case

1. Create tables on demand

Field nameField MeaningField typeconstraint conditionConstraint Keys
idID Unique IdentificationintPrimary key and self-growingPRIMARY KEY,AUTO_INCREMENT
nameFull namevarchar(10)Not empty and uniqueNOT NULL,UNIQUE
ageAgeintGreater than 0 and less than or equal to 120CHECK
statusstatechar(1)Default 1 if no value is specifiedDEFAULT
genderGenderchar(1)nothing

2. Functional implementation

--Constraint table creation
create table user(
    id int primary key  auto_increment comment 'Primary key',
    name varchar(10) not null unique comment 'Full name',
    age int check (age>0 && age<=120) comment 'Age',
    status char(1) default '1' comment 'state',
    gender char(1) comment 'Gender'
)comment 'User table';
--insert data
insert into user(name,age,status,gender) value ('Tom1',19,'1','male'),('Tom2',25,'0','male')

3. Graphical Interface Creation Table

Right-click tables_New_Table under test database

3. Foreign Key Constraints

1. Concepts

Connect data between two tables to ensure data consistency and integrity.

Note: At the database level, these two tables do not have foreign key associations, so data consistency and integrity cannot be guaranteed.

2. Grammar

1. Create foreign keys directly

CREATE TABLE table name

Field name data type,

‚Äč ...

[CONSTRAINT] [Foreign Key Name] FOREIGN KEY (Foreign Key Field Name) REFERENCES main table (Primary Table Column Name)

);

(2) Add foreign keys

ALTER TABLE table name ADD CONSTRAINT foreign key name FOREIGN KEY (foreign key field name) REFERENCES main table (main table column name);

--Data preparation
--1.Create Department Table
create table dept(
    id int auto_increment comment 'ID' primary key ,
    name varchar(50) not null comment 'Department Name'
)comment 'Department table'
--2.Enter departmental data
insert into dept(id,name) values (1,'R&D Department'),(2,'Marketing Department'),(3,'Finance Department'),(4,'Sales Department'),(5,'Chief Executive');
--3.Create employee table
create table emp(
    id int auto_increment comment 'ID' primary key,
    name varchar(50) not null comment 'Full name',
    age int comment 'Age',
    job varchar(20) comment 'position',
    salary int comment 'salary',
    entrydate date comment 'Enrollment Time',
    managerid int comment 'Direct Leadership ID',
    dept_id int comment 'department ID'
)comment 'Employee Sheet';
--4.Enter employee data
insert into emp(id,name,age,job,salary,entrydate,managerid,dept_id) values
    (1,'Jin Yong',66,'CEO',20000,'2000-01-01',null,5),
    (2,'Zhang Wuji',20,'project manager',12500,'2005-12-05',1,1),
    (3,'Yang Xiao',33,'Development',8400,'2000-11-03',2,1),
    (4,'Xiangr',48,'Development',11500,'2002-02-05',2,1),
    (5,'Zhao Min',22,'Development',10500,'2004-09-07',3,1),
    (6,'Xiao Zhao',19,'Trainee',6600,'2004-10-12',2,1);
--Add Foreign Key
alter table emp add constraint fk_emp_dept_id foreign key (dept_id) references dept(id);

(3) Delete foreign keys

ALTER TABLE table name DROP FOREIGN KEY foreign key name;

--Delete foreign keys
alter table emp drop foreign key fk_emp_dept_id;

3. Delete/update behavior of foreign key constraints

1 Behavior

behaviorExplain
NO ACTIONWhen deleting/updating a corresponding record in the parent table, first check if the record has a corresponding foreign key, if deletion/updating is not allowed. (same as RESTRICT)
RESTRICTWhen deleting/updating a corresponding record in the parent table, first check if the record has a corresponding foreign key, if deletion/updating is not allowed. (consistent with NO ACTION)
CASCADEWhen deleting/updating a corresponding record in the parent table, first check if the record has a corresponding foreign key, and if so delete/update the record in the child table
SET NULLWhen deleting/updating a corresponding record in the parent table, check to see if the record has a corresponding foreign key. If so, set the value of the foreign key to null in the wordtable (which requires that the foreign key allow null)
SET DEFAULTWhen the parent table changes, the wordtable sets the foreign key column to a default value (innodb does not support it)

(2) Grammar

ALTER TABLE table name ADD CONSTRAINT foreign key name FOREIGN KEY (foreign key field name) REFERENCES main table (main table column name) ON UPDATE CASCADE ONDELETE CASCADE;

Keywords: Javascript Front-end MySQL TypeScript

Added by Gen-ik on Mon, 07 Mar 2022 20:13:22 +0200