The concept, function, characteristics, creation and deletion of MySQL constraints

1 The role of constraints

1.1 Generalization

• not null（ Non empty constraint ）
• unique（ Uniqueness constraint ）
• primary key（ Primary key constraint ）
• foreign key（ Foreign key constraints ）
• check（ Check constraint ）
• default（ Default constraint ）

1.2 classification

1.2.1 Non empty constraint

1.2.1.1 effect

Limit a field / The value of a column cannot be empty

1.2.1.2 keyword

NOT NULL

1.2.1.3 characteristic

• By default, all types of values can be NULL
• Non empty constraints can only appear on columns of table objects , Only one column can be qualified as non empty , Cannot combine non empty
• A table can have many columns, which are limited to non empty
• An empty string " It's not equal to NULL,0 Also is not equal to NULL

1.2.1.4 Add non empty constraints

1.2.1.4.1 Add constraints when creating tables

 CREATE TABLE test1 (
	id INT  NOT NULL,--  Add a non NULL constraint when creating a table 
	last_name VARCHAR ( 20 ) NOT NULL,--  Add a non NULL constraint when creating a table 
	email VARCHAR ( 25 ),
	salary DECIMAL ( 10, 2 ) 
)

1.2.1.4.2 Add constraints when modifying tables

--  Add non empty constraints when modifying tables 
ALTER TABLE test1 MODIFY email VARCHAR ( 25 ) NOT NULL;

1.2.1.5 Delete constraints

--  Delete non empty constraints when modifying tables 
ALTER TABLE test1 MODIFY email VARCHAR ( 25 );

1.2.2 Uniqueness constraint

1.2.2.1 effect

Used to restrict a field / The value of a column cannot be repeated .

1.2.2.2 keyword

UNIQUE

1.2.2.3 characteristic

• The same table can have multiple unique constraints
• A unique constraint can be a unique value for a column , You can also combine multiple columns to have unique values
• The uniqueness constraint allows column values to be empty
• When creating unique constraints , If you don't name a unique constraint , The default is the same as the column name
• Mysql A unique index will be created by default for columns with unique constraints

1.2.2.4 Add unique constraints

1.2.2.4.1 Add constraints when creating tables

CREATE TABLE test2 (
	--  Column level constraints 
	id INT UNIQUE,--  Add unique constraints when creating tables 
	last_name VARCHAR ( 20 ),
	email VARCHAR ( 25 ),
	salary DECIMAL ( 10, 2 ),
	--  Table level constraints 
	CONSTRAINT uk_test2_email UNIQUE ( email )
)

--  The uniqueness constraint of composition ( Only when the two fields are the same can they conflict )
CREATE TABLE test3( 
	id INT,
	last_name VARCHAR ( 20 ),
	email VARCHAR ( 25 ),
	salary DECIMAL ( 10, 2 ),
	--  Table level constraints 
	CONSTRAINT uk_test2_id_email UNIQUE ( id,email )
)

1.2.2.4.2 Add constraints when modifying tables

--  Add unique constraints 
--  Mode one 
ALTER TABLE test2 ADD CONSTRAINT UNIQUE(last_name);
--  Add composite constraints 
ALTER TABLE test3 ADD CONSTRAINT uk_test2_last_name_email UNIQUE(last_name,email);
--  Mode two 
ALTER TABLE test2 MODIFY salary DECIMAL ( 10, 2 ) UNIQUE;

1.2.2.5 Delete constraints

ALTER TABLE < Table name > DROP INDEX < Unique constraint name >;

1.2.2.5.1 matters needing attention

• A unique index will also be automatically created on the column to which the uniqueness constraint is added
• Deleting a unique constraint can only be deleted by deleting a unique index
• You need to specify a unique index name when deleting , The unique index name is the same as the unique constraint name
• If you do not specify a name when creating a unique constraint , If it's a single column , The default is the same as the column name ; If it's a composite column , Then default and （） The first column in the list has the same name . You can also customize the uniqueness constraint name

1.2.2.5.2 Code

--  Look at the index 
SHOW INDEX FROM test2;
--  Delete unique constraint 
ALTER TABLE test2 DROP INDEX salary;

1.2.3 Primary key constraint

1.2.3.1 effect

Used to uniquely identify a row of records in the table .

1.2.3.2 keyword

primary key

1.2.3.3 characteristic

• A table can have at most one primary key constraint , You can create a primary key constraint at the column level , You can also create... At the table level
• The primary key constraint corresponds to one or more columns in the table （ Composite primary key ）
• If it is a multi column composite primary key constraint , Then none of these columns are allowed to be null , And the combined value is not allowed to repeat
• MySQL The primary key name of is always PRIMARY, Even if you name the primary key constraint name, it's useless
• When creating a primary key constraint , By default, the system will establish a corresponding primary key index on the column or column combination （ That can be queried according to the primary key , Query according to the primary key , More efficient ）. If the primary key constraint is deleted , The index corresponding to the primary key constraint is automatically deleted
• One thing to note is that , Do not modify the value of the primary key field . Because the primary key is the unique identification of the data record , If you modify the value of the primary key , It may destroy the integrity of the data

1.2.3.4 Add primary key constraint

1.2.3.4.1 Add constraints when creating tables

	CREATE TABLE test4 ( 
	id INT PRIMARY KEY, --  Column level constraints 
	last_name VARCHAR ( 15 ),
	salary DECIMAL ( 10, 2 ), 
	email VARCHAR ( 25 ) 
	);
	
	CREATE TABLE test5 ( 
	id INT, 
	last_name VARCHAR ( 15 ),
	salary DECIMAL ( 10, 2 ), 
	email VARCHAR ( 25 ),
	--  Table level constraints 
	CONSTRAINT PRIMARY KEY ( id )
	);
	
	CREATE TABLE test6 ( 
	id INT, 
	last_name VARCHAR ( 15 ),
	salary DECIMAL ( 10, 2 ), 
	email VARCHAR ( 25 ),
	--  Composite primary key , The two columns are regarded as a whole as the primary key 
	CONSTRAINT  PRIMARY KEY ( id,last_name )
	);

1.2.3.4.2 Add constraints when modifying tables

	ALTER TABLE test7 ADD PRIMARY KEY(id);

1.2.3.5 Delete constraints

	ALTER TABLE test7 DROP PRIMARY KEY;

1.2.4 On the column

1.2.4.1 effect

The value of a field increases automatically

1.2.4.2 keyword

auto_increment

1.2.4.3 characteristic

• A table can only have at most one self growing column
• When a unique identifier or sequential value needs to be generated , Self growth can be set
• The column of the self growing column constraint must be a key column （ Primary key column , Unique key column ）
• The data type of a column with a self increasing constraint must be an integer type
• If the auto increment column specifies 0 and null, It will automatically increase based on the current maximum ; If the auto increment column manually specifies a specific value , Direct assignment to a specific value .

1.2.4.4 Add auto increment column

1.2.4.4 .1 Add self incrementing columns when creating tables

CREATE TABLE test8 ( 
id INT PRIMARY KEY AUTO_INCREMENT, --  Column level constraints 
last_name VARCHAR ( 15 ), 
salary DECIMAL ( 10, 2 ), 
email VARCHAR ( 25 ) 
);

1.2.4.4 .1 Add auto increment column when modifying the table

ALTER TABLE test9 MODIFY id INT AUTO_INCREMENT;

1.2.4.5 Delete auto increment column

ALTER TABLE test9 MODIFY id INT;

1.2.5 Foreign key constraints

1.2.5.1 effect

Limit the referential integrity of a field in a table .

1.2.5.2 keyword

foreign key

1.2.5.3 characteristic

• From the foreign key column of the table , Must quote / Columns that refer to the primary key or unique constraint of the main table , Because of being dependent / The referenced value must be unique
• When creating a foreign key constraint , If you don't name the foreign key constraint , The default name is not a column name , Instead, it automatically generates a foreign key name （ for example student_ibfk_1;), You can also specify a foreign key constraint name
• establish （CREATE） If you specify a foreign key constraint when using the table , Create the main table first , Then create the slave table
• When deleting a table , Delete from the table first （ Or delete the foreign key constraint first ）, Delete the main table
• When the records of the master table are referenced from the slave table , Records in the main table will not be allowed to be deleted , If you want to delete data , You need to delete the data that depends on this record from the table first , Then you can delete the data of the main table
• stay “ From the table ” Specify foreign key constraints in , And a table can establish multiple foreign key constraints
• The foreign key column of the slave table and the column name of the main table can be different , But the data type must be the same , The logical meaning is consistent . If the types are different , When creating a child table , There will be errors “ERROR 1005(HYo00):Can’t create table’database.tablename’(errno:150)”. for example ： They all represent the department number , All are int type .
• When creating a foreign key constraint , By default, the system will establish the corresponding common index on the column . But the index name is the column name , Is not the constraint name of a foreign key .（ It is very efficient to query according to foreign keys ）
• After deleting the foreign key constraint , The corresponding index must be manually deleted

1.2.5.4 Add a foreign key constraint

1.2.5.4.1 Add constraints when creating tables

--  Create the main table  
CREATE TABLE dept ( 
	dept_id INT PRIMARY KEY,
	dept_name VARCHAR ( 15 ) 
 );
 --  Create from table 
 CREATE TABLE emp ( 
	emp_id INT PRIMARY KEY AUTO_INCREMENT,
	emp_name VARCHAR ( 15 ),
	department_id INT,
	--  Foreign key link 
	CONSTRAINT fk_emp_dept_id FOREIGN KEY(department_id) REFERENCES dept(dept_id)
 );

1.2.5.4.2 Add constraints when modifying tables

 --  Create the main table  
CREATE TABLE dept2 ( 
	dept_id INT PRIMARY KEY,
	dept_name VARCHAR ( 15 ) 
 );
 --  Create from table 
 CREATE TABLE emp2 ( 
	emp_id INT PRIMARY KEY AUTO_INCREMENT,
	emp_name VARCHAR ( 15 ),
	department_id INT
 );
 --  Add foreign keys 
 ALTER TABLE emp2 ADD CONSTRAINT fk_emp2_dept_id FOREIGN KEY(department_id) REFERENCES dept2(dept_id);

1.2.5.5 Delete foreign key constraint

 --  Create the main table  
CREATE TABLE dept2 ( 
	dept_id INT PRIMARY KEY,
	dept_name VARCHAR ( 15 ) 
 );
 --  Create from table 
 CREATE TABLE emp2 ( 
	emp_id INT PRIMARY KEY AUTO_INCREMENT,
	emp_name VARCHAR ( 15 ),
	department_id INT
 );
 --  Add foreign keys 
 ALTER TABLE emp2 ADD CONSTRAINT fk_emp2_dept_id FOREIGN KEY(department_id) REFERENCES dept2(dept_id);
 --  Delete foreign key 
 ALTER TABLE emp2 DROP FOREIGN KEY fk_emp2_dept_id; 
 
 --  View the general index corresponding to the foreign key constraint 
 SHOW INDEX FROM emp2;
 --  Delete index （ Indexes are created with the creation of foreign keys , The name is the foreign key constraint name ）
 ALTER TABLE emp2 DROP INDEX fk_emp2_dept_id;

1.2.6 Check constraint （Mysql8.0 Support ）

1.2.6.1 effect

Check whether the value of a field is a symbol xx requirement , Generally refers to the range of values .

1.2.6.2 keyword

CHECK

1.2.6.3 Add check constraints

1.2.6.3.1 Add constraints when creating tables

 CREATE TABLE test10(
id INT,
last_name VARCHAR(15),
salary DECIMAL(10,2) CHECK(salary > 2000)
 );

1.2.7 Default constraint

1.2.7.1 effect

Give a field / Specify a default value for a column , Once the default value is set , When inserting data , If this field has no explicit assignment , The default value is assigned .

1.2.7.2 keyword

DEFAULT

1.2.7.3 Add check constraints

1.2.7.3.1 Add constraints when creating tables

 CREATE TABLE test11(
	id INT,
	last_name VARCHAR(15),
	salary DECIMAL(10,2) DEFAULT(3000)
 );

1.2.7.3.2 Add constraints when modifying tables

 CREATE TABLE test13(
	id INT,
	last_name VARCHAR(15),
	salary DECIMAL(10,2)
 );
 ALTER TABLE test13 MODIFY salary DECIMAL(10,2) DEFAULT(3000);

2 How to add constraints

• create table Add constraints when
• altert table Add constraints when 、 Delete constraints

3 How to view constraints in a table

DESC  Table name 

4 Constraint level

example ：