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# The first way to add 
CREATE TABLE student1(
`id` INT PRIMARY KEY,
`name` VARCHAR(50)
``
)

--> Display table structure 
mysql> desc student1;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | NO   | PRI | NULL    |       |
| name  | varchar(50) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

（2） The second way to add ： The advantage of this way is , You can create a union primary key

# The second way to add : The advantage of this way is , You can create a union primary key 
#(1)
CREATE TABLE student2(
`id` INT,
`name` VARCHAR(50),
`classId` INT,
PRIMARY KEY(id,classId)
);

--> Display table structure 
mysql> desc student2;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id      | int         | NO   | PRI | NULL    |       |
| name    | varchar(50) | YES  |     | NULL    |       |
| classId | int         | NO   | PRI | NULL    |       |
+---------+-------------+------+-----+---------+-------+

（3） The third way to add ：

# The third way to add 
CREATE TABLE student3(
`id` INT,
`name` VARCHAR(50)
);
ALTER TABLE student3 ADD PRIMARY KEY(id);


--> Display table structure 
mysql> desc student3;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | NO   | PRI | NULL    |       |
| name  | varchar(50) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

1.2、 Unique constraint （unique）

characteristic ： Data cannot be repeated

CREATE TABLE student4(
`id` INT PRIMARY KEY,
`name` VARCHAR(50) UNIQUE
)

--> Display table structure 
mysql> desc student4;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | NO   | PRI | NULL    |       |
| name  | varchar(50) | YES  | UNI | NULL    |       |
+-------+-------------+------+-----+---------+-------+

1.3、 Auto grow Columns （auto_increment）

Be careful ： Add an auto growing number to the primary key , Columns can only be of integer type

# Auto grow Columns ：auto_increment
CREATE TABLE student5(
`id` INT PRIMARY KEY AUTO_INCREMENT,
`name` VARCHAR(50)
)

--> final result ：
mysql> SELECT * FROM student5;
+----+----------+
| id | NAME     |
+----+----------+
|  1 | zhangsan |
|  2 | lisi     |
|  3 | wangwu   |
+----+----------+

02、 Domain integrity

The role of domain integrity ： Limit the correct data of this unit , Do not compare other cells in this column

The field represents the current cell

Domain integrity constraints ：

（1） data type ： value type 、 The date type 、 String type

（2） Non empty constraint （not null）

（3） Default constraint （default）

2.1、 data type

（1） value type

（2） String type

（3） Date and time numerical type

2.2、 Non empty constraint ：NOT NULL

# Non empty constraint ：not null
CREATE TABLE student6(
`Id` INT PRIMARY KEY,
`Name` VARCHAR(50) NOT NULL,
`Sex` VARCHAR(10)
);
INSERT INTO student6 VALUES(1,'tom',NULL);

--> Display table structure 
mysql> SELECT * FROM student6;
+----+------+------+
| Id | NAME | Sex  |
+----+------+------+
|  1 | tom  | NULL |
+----+------+------+

2.3、 Default constraint :default

# Default constraint ：default
CREATE TABLE student7(
`Id` INT PRIMARY KEY,
`Name` VARCHAR(50) NOT NULL,
`Sex` VARCHAR(10) DEFAULT ' male '  # The default value is set to ' male '
);

INSERT INTO student7 VALUES(1,'tom',' Woman ');
INSERT INTO student7 VALUES(2,'jerry',DEFAULT);


--> Display table structure 
mysql> SELECT * FROM student7;
+----+-------+------+
| Id | NAME  | Sex  |
+----+-------+------+
|  1 | tom   |  Woman    |
|  2 | jerry |  male    |
+----+-------+------+

03、 Citation integrity （ Referential integrity ）

Foreign key constraints ：foreign key

# Foreign key constraints ：foreign key
CREATE DATABASE mydb;
CREATE TABLE student8(
sid INT PRIMARY KEY,
NAME VARCHAR(50) NOT NULL,
sex VARCHAR(10) DEFAULT ' male '
);
INSERT INTO student8 
VALUES
(1001,' Zhang San ',' male '),
(1002,' Li Si ',' male '),
(1003,' Wang Wu ',' Woman ');


# Add foreign key mode one ：
CREATE TABLE score1(
	id INT,
	score INT,
	sid INT , --  The data type of the foreign key column must be consistent with that of the primary key 
	CONSTRAINT fk_score_sid FOREIGN KEY (sid) REFERENCES student8(sid)
);
# Add foreign key mode 2 ：
CREATE TABLE score2(
	id INT,
	score INT,
	sid INT --  The data type of the foreign key column must be consistent with that of the primary key 
);
ALTER TABLE score2 ADD CONSTRAINT fk_stu_score FOREIGN KEY(sid) REFERENCES student8(sid);


--> Display table structure ： Add foreign key mode one 
mysql> DESC score1;
+-------+------+------+-----+---------+-------+
| FIELD | TYPE | NULL | KEY | DEFAULT | Extra |
+-------+------+------+-----+---------+-------+
| id    | INT  | YES  |     | NULL    |       |
| score | INT  | YES  |     | NULL    |       |
| sid   | INT  | YES  | MUL | NULL    |       |
+-------+------+------+-----+---------+-------+


--> Display table structure ： Add foreign key mode 2 
mysql> DESC score2;
+-------+------+------+-----+---------+-------+
| FIELD | TYPE | NULL | KEY | DEFAULT | Extra |
+-------+------+------+-----+---------+-------+
| id    | INT  | YES  |     | NULL    |       |
| score | INT  | YES  |     | NULL    |       |
| sid   | INT  | YES  | MUL | NULL    |       |
+-------+------+------+-----+---------+-------+

04、 The relationship between tables

1、 one-on-one

for example t_person Table and t_card surface , People and ID cards . This situation requires a master-slave relationship , Who is the main table , Who is from the table . People can have no ID card , But the ID card must have talents , So people are the main watch , And the ID card is from the table . There are two schemes for designing slave tables ：

（1） stay t_card Add foreign key columns to the table （ relative t_user surface ）, And add a unique constraint to the foreign key ;

（2） to t_card Add a foreign key constraint to the primary key of a table （ relative t_user surface ）, namely t_card The primary key of a table is also a foreign key .

2、 One to many

The most common is one to many ！ One to many and many to one , From what angle can we see this .t_user and t_section The relationship between , from t_user It's one to many , And from t_section From the perspective of many to one ！ In this case, foreign keys are created in multiple parties ！

3、 Many to many

for example t_stu and t_teacher surface , That is, a student can have more than one teacher , And a teacher can have more than one student . In this case, you usually need to create an intermediate table to handle many to many relationships . For example, create another table t_stu_tea surface , Give two foreign keys , A relatively t_stu Table foreign key , Another relative t_teacher Table foreign key .