mysql给一张表添加外键的4种方法

create table score(
cscore int(11),
st_id int(50) references student(id),
cs_id int(30) references classes(id),
primary key(st_id,cs_id)
);

create table score(
cscore int(11),
st_id int(50),
cs_id int(30),
primary key(st_id,cs_id),
FOREIGN KEY (st_id) REFERENCES student(id),
FOREIGN KEY (cs_id) REFERENCES classes(id)
);

create table score(
cscore int(11),
st_id int(50),
cs_id int(30),
primary key(st_id,cs_id),
CONSTRAINT `FK_ID_ST` FOREIGN KEY (st_id) REFERENCES student(id),
CONSTRAINT `FK_ID_CS` FOREIGN KEY (cs_id) REFERENCES classes(id)
);

alter table 表名 add constraint FK_ID foreign key(你的外键字段名) REFERENCES 外表表名(对应的表的主键字段名);

CREATE TABLE tb
(
id INT IDENTITY(1,1) PRIMARY KEY,
name VARCHAR(20)
)

*CREATE TABLE table_test(
id INT NOT NULL,
name VARCHAR(20) NOT NULL,
address VARCHAR(20),
PRIMARY KEY(id)
);

CREATE TABLE table_test(
user_id INT NOT NULL,
user_name VARCHAR(20) NOT NULL,
user_address VARCHAR(20),
PRIMARY KEY (user_id, user_name)
);*

alter table 表名 add primary key（字段列表）

ALTER TABLE Employees
ADD CONSTRAINT PK_EmployeesID PRIMARY KEY (EmployeeID)

ALTER TABLE （表名） ADD id INT(16) NOT NULL
PRIMARY KEY AUTO_INCREMENT FIRST;

alter table（表名） modify id integer auto_increment

alter （表名） DROP PRIMARY KEY

# 更新模式
alter table my_student add foreign key(class_id)
references my_class(class_id)
-- 约束条件
on update cascade
on delete set null;
-- 更新主表
update my_class set class_id = 4 where class_id = 2;
-- 执行更新主表操作后，从表也随便主表的变化而变化了

#删除模式
delete from my_class where class_id = 4;
-- 执行以上操作后，学生表my_student class_id原来值为4字段现在全部为空