MySQL constraints

1.MySQL constraint

Concept ：
Constrained English ：constraint
Constraints are actually constraints on the data in the table
effect ：
The purpose of adding constraints in the table design is to ensure the records in the table Integrity and effectiveness , For example, the value of some columns in the user table cannot be empty , Some column values cannot be repeated .
classification ：

• Primary key constraint (primary key)
• Self increasing constraint (auto_increment)
• Non empty constraint (not null)
• Uniqueness constraint (unique)
• Default constraint (default)
• Zero fill constraint (zerofill)
• Foreign key constraints (foreign key)

1.1 Primary key constraint

Concept ：

• MySQL The primary key constraint is what one or more columns do , Its value uniquely identifies each row in the table , Convenient in RDBMS Find a line as soon as possible .
• A primary key constraint is equivalent to a unique constraint + A combination of nonnull constraints , Primary key constraint columns are not allowed to be duplicate , Null values are not allowed .
• Only one primary key is allowed per table
• Keywords are ：primary key

Add single column primary key ：

CREATE TABLE bianbian.emp1(

eid int PRIMARY KEY,
name VARCHAR(20),
deptId INT,
salary DOUBLE
);

CREATE TABLE bianbian.emp2(
eid INT,
name VARCHAR(20),
deptId INT,
salary DOUBLE,
CONSTRAINT pk1 PRIMARY KEY(eid) --CONSTRAINT pk1  It can be omitted 
);

--  The function of primary key 
--  Primary key constraint , The primary key column is not empty and duplicate 
INSERT into emp2(eid,name,deptId,salary) VALUES(1001,' Zhang San ',10,5000);
--  Repeat validation 
INSERT into emp2(eid,name,deptId,salary) VALUES(1001,' Li Si ',10,6000);
--  Can't be empty 
INSERT into emp2(eid,name,deptId,salary) VALUES(NULL,' Li Si ',10,6000);

Combined the primary key ：
The so-called federated primary key , The primary key is composed of multiple fields in a table .
Be careful ：
1. When the primary key is composed of multiple fields , You cannot declare a primary key constraint directly after a field name
2. A table can only have one primary key , A federated primary key is also a primary key

Code ：

CREATE TABLE bianbian.emp3(
name VARCHAR(20),
deptId INT,
salary DOUBLE,
PRIMARY KEY(name,deptId)
);

Running results ：

The primary key function ：

--  The marked column of the union primary key cannot be duplicate or empty 
INSERT INTO emp3 VALUES(' Zhang San ',10,5000);
INSERT INTO emp3 VALUES(' Zhang San ',20,5000);
INSERT INTO emp3 VALUES(' Wang Wu ',20,5000);
-- name and deptId  It is the same as the original content , Will report a mistake 
INSERT INTO emp3 VALUES(' Zhang San ',20,5000);
--  An error is also reported if it is empty 
INSERT INTO emp3 VALUES(NULL,20,5000);
INSERT INTO emp3 VALUES(' Extrajudicial ',5000);

Code ：

CREATE TABLE bianbian.emp4(
eid INT,
name VARCHAR(20),
deptId INT,
salary DOUBLE
);
ALTER TABLE emp4 ADD PRIMARY key(eid);

Delete primary key ：

--  There is no single primary key or joint primary key 
ALTER table emp4 DROP PRIMARY KEY

1.2 Self growth constraints

Code ：

--  Self growth constraints 
CREATE TABLE bianbian.t_user1(
id int PRIMARY KEY auto_increment,
name VARCHAR(20)
);
INSERT INTO t_user1 VALUES(NULL,' Zhang San ');
INSERT INTO t_user1(name) VALUES(' Li Si ');

Code ：

--  Self growth constraints 
--  Mode one ： Specify... When creating a table 
CREATE TABLE bianbian.t_user2(
id int PRIMARY KEY auto_increment,
name VARCHAR(20)
)auto_increment = 100; -- id from 100 Start 
--  Mode two ： After creating the table, specify 
CREATE TABLE bianbian.t_user3(
id int PRIMARY KEY auto_increment,
name VARCHAR(20)
);
ALTER TABLE t_user3 auto_increment = 200 -- id from 200 Start 

DELETE FROM t_user1; -- DELETE After deleting data , Self growth is still the last value plus 1
TRUNCATE t_user1; -- TRUNCATE After deleting data , Start with the default assignment 

1.3 Non empty constraint

CREATE TABLE bianbian.t_user7(
id INT,
name VARCHAR(20)  ,  
address VARCHAR(20) 
);

ALTER TABLE t_user7 MODIFY name VARCHAR(20) NOT NULL; --  Specify a non NULL constraint 
ALTER TABLE t_user7 MODIFY address VARCHAR(20) NOT NULL;--  Specify a non NULL constraint 

INSERT INTO t_user7(id) VALUES(1001);  --  Report errors 
ALTER TABLE t_user7 MODIFY address VARCHAR(20);--  Delete non empty constraints 

1.4 Unique constraint

Code ：

-- 1.  Add unique constraints  -  The way 1- Specify... When creating a table 
CREATE TABLE bianbian.t_user8(
id INT,
name VARCHAR(20),  
phone_number INT UNIQUE  --  Specify a unique constraint  
);
-- 2. Add unique constraints  -  The way 2 Specify... When creating a table 
CREATE TABLE bianbian.t_user9(
id INT,
name VARCHAR(20),  
phone_number INT   --  Specify a unique constraint  
);
ALTER TABLE t_user9 ADD CONSTRAINT phone_unique UNIQUE(phone_number);
-- 3. Delete unique constraint 
--  Format ： ALTER TABLE  Table name  DROP INDEX  Unique constraint name 
ALTER TABLE t_user9 DROP INDEX phone_unique;

1.5 Default constraint

Code ：

CREATE TABLE bianbian.t_user11(
id INT,
name VARCHAR(20),  
address VARCHAR(20)
);
ALTER TABLE t_user11 MODIFY address VARCHAR(20) DEFAULT ' Shenzhen '; --  Add default constraint 
ALTER TABLE t_user11 MODIFY address VARCHAR(20) DEFAULT NULL;   --  Delete default constraint 

Reference resources

• https://www.bilibili.com/video/BV1iF411z7Pu?p=39