SQL 约束 索引

--约束constraint
--not null 非空约束
create table demo01(
empno number(4),
ename varchar2(10) not null,
job varchar2(10)
)

insert into demo01 values(1234,' ','SALES');
select * from demo01;

--unique 唯一键
create table demo02(
empno number(4) unique,
ename varchar2(10),
job varchar2(8)
)
select * from demo02;
insert into demo02 values(1234,'zhangsan','SALES');
insert into demo02 values(1234,'lisi','SALES');
insert into demo02 values(null,'wangwu','SALES');
insert into demo02 values(null,'zhaoliu','SALES');

--非空&唯一
create table demo03(
empno number(4) not null unique,
ename varchar2(10),
job varchar2(8)
)
insert into demo03 values(1234,'zhangsan','SALES');
insert into demo03 values(null,'lisi','SALES');

--自定义检查约束
create table demo04(
empno number(4),
ename varchar2(10),
job varchar2(8),
age number(3) check(age>0 and age<200),
gender varchar2(3) check(gender='男' or gender='女')
)

insert into demo04 values(1234,'zhangsan','SALES',56,'男')
insert into demo04 values(1234,'zhangsan','SALES',44,'详')

--创建约束时，为约束起名 constraint
create table demo05(
empno number(4),
ename varchar2(10),
job varchar2(8),
age number(3) constraint ck_age check(age>0 and age<200)
)

insert into demo05 values(1234,'zhangsan','SALES',300);

--在添加完列后，还可以添加约束
--除了not null不可以
create table demo06(
empno number(4),
ename varchar2(10),
age number(3),
constraint uq_empno unique(empno),
constraint ck_age2 check(age>0 and age<200)
)

insert into demo06 values(1234,'zhangsan');
insert into demo06 values(1234,'lisi');

--主键约束
--为了保证该列的数据能够证明行记录在表中是唯一的
--主键约束从形式看，类同于（非空和唯一）约束
create table demo07(
empno number(4) primary key,
ename varchar2(10),
age number(3)
)

insert into demo07 values(1234,'zhangsan',18);
insert into demo07 values(1234,'lisi',18);
insert into demo07 values(null,'wangwu',20);

--在列之后添加
create table demo08(
empno number(4),
ename varchar2(10),
age number(3),
constraint pk_empno primary key(empno)
)

-- 一张表只有一个主键
create table demo09(
empno number(4) primary key,
ename varchar2(10) primary key,
age number(3)
--constraint pk_empno primary key(empno)
)

-- 一张表虽然只有一个主键，但可以选择多个列，作为联合主键
create table demo09(
empno number(4) ,
ename varchar2(10),
age number(3),
constraint pk_empno2 primary key(empno,ename)
)

insert into demo09 values(1234,'zhangsan',18);
insert into demo09 values(1234,'lisi',20);
insert into demo09 values(2345,'zhangsan',20);

insert into demo09 values(1234,null,18);

--外键foreign key
--外键用来描述两张表之间的关系，必须保证外键列的值必须为参考列的值中的一个
create table t_class(
classid number(2) primary key,
classname varchar2(20) not null,
classnum varchar2(30)
)

insert into t_class values(10, 'java班','教室6');
insert into t_class values(20, '大数据班','教室3');
insert into t_class values(30, 'python班','教室4');

delete from t_class where classid = 20;

create table t_student(
stuid number(8) primary key,
stuname varchar2(30) not null,
stuphone varchar2(11),
classid varchar2(2) references t_class(classnum)
)

insert into t_student values(11111111,'方子恒',110,10);
insert into t_student values(11111112,'马子明',120,20);
insert into t_student values(11111113,'王福禄',130,10);

select * from t_class;
select * from t_student;

--存在外键的表为从表，被依赖的表为主表
--外键不一定为主表的主键，也可以为主表的唯一键

--约束除了在建表时，可以添加约束外，还可以修改表时修改约束
alter table demo07 add constraint uq_demo07_ename unique(ename);

alter table demo07 drop constraint uq_demo07_ename

--索引：
create index idx_emp_ename_job on emp(ename,job)

select * from emp where ename = 'SMITH' and job= 'XXX';

--只有添加索引的列，通过该列进行查询时，速度才可以变快

create table demo110(
id number(5) primary key,
name varchar2(20) not null,
nickname varchar2(30) unique
)