创建数据库表
CREATE TABLE student(
sno int (6),
sname VARCHAR(10),
sex CHAR(1),
age INT(2),
enterdate date,
classname VARCHAR(10),
email VARCHAR(15)
);
– 查看表的结构
desc student
– 查看表的数据
SELECT * FROM student
DML
– 查询表的数据
SELECT * FROM student
– insert
insert into student values(1,‘张三’,‘男’,23,‘2021/2/27’,‘一班’,‘zhs@.com’);
insert into student values(2,‘李四’,‘男’,24,‘2021/2/27’,‘一班’,‘lis@.com’);
insert into student (sno,sname,sex,classname) values (3,‘王五’,‘男’,‘二班’)
– update
update student set sex = ‘女’ where sno = 2
update student set sex = ‘男’,age = 25,classname = null where sno = 2
– delete
delete FROM student WHERE classname = ‘二班’
– 删除表中的所有数据
delete FROM student
truncate table student – 速度快，无法回滚 DDL语言 效率高
DDL
– DDL creat alter drop
– DML insert update delete
– 查询数据
select * from student
– 查询表的结构
desc student
– 修改表的数据
– 增加一列
alter table student add score double(5,2);
alter table student add score double(5,2) first
alter table student add score double(5,2) after enterdate;
-- 修改一列
alter table student modify score float(4,1);
alter table student change score score1 double(4,1);
-- 删除一列
alter table student drop score;
– 删除表
drop table student
非外键约束
– 列级约束
CREATE TABLE student(
sno int (6) primary key auto_increment,
sname VARCHAR(10) not null,
sex CHAR(1) default ‘男’ check(sex = ‘男’ or sex = ‘女’),
age INT(2) check(age>=0 and age<=50),
enterdate date,
classname VARCHAR(10),
email VARCHAR(15)
);
– 表级约束
CREATE TABLE student(
sno int (6) auto_increment,
sname VARCHAR(10) not null,
sex CHAR(1) default’男’,
age INT(2),
enterdate date,
classname VARCHAR(10),
email VARCHAR(15),
constraint pk_stu primary key(sno),
constraint ck_stu_sex check(sex = ‘男’ or sex = ‘女’),
constraint ck_stu_age check(age>=0 and age<=50),
constraint ck_stu_email unique(email)
);
CREATE TABLE student(
sno int (6),
sname VARCHAR(10) not null,
sex CHAR(1) default ‘男’, – not null 和 default 只能使用列级约束
age INT(2),
enterdate date,
classname VARCHAR(10),
email VARCHAR(15)
);
alter table student add constraint pk_stu primary key(sno);
alter table student add constraint ck_stu_sex check(sex = ‘男’ or sex = ‘女’);
alter table student add constraint ck_stu_age check(age>=0 and age<=50);
alter table student add constraint ck_stu_email unique(email);
alter table student modify sno int(6) auto_increment
外键约束
create table t_student(
sno int(6)primary key auto_increment,
sname varchar(10) not null,
age int(3),
score double(4,1),
classno int(4)
– constraint fk_stu_classno foreign key(classno) references t_class(cno)
);
– 外键
alter table t_student add constraint fk_stu_classno foreign key(classno) references t_class(cno)
外键策略
– 1.NO Action:
delete from t_class cno=2
update t_student set classno = null where class =2
– 2.cascade级联
– 加外键alter table t_student add constraint fk_stu_classno foreign key(classno) references t_class(cno)
– 删外键alter table t_student drop FOREIGN KEY fk_stu_classno
– 级联
alter table t_student add constraint fk_stu_classno foreign key(classno) references t_class(cno) on update cascade on delete cascade
– 3.set null
alter table t_student add constraint fk_stu_classno foreign key(classno) references t_class(cno) on update set null on delete set null
)
