常用SQL語句

主要總結mysql一些常用知識點

[常用命令]

1、查看數據庫

show database;

2、創建數據庫

create database database_name;

3、切換數據庫

use database_name;

4、查看某數據庫中所有的數據表

show table;

5、創建數據表

CREATE TABLE my_table (
    name VARCHAR(20),
    owner VARCHAR(20),
    species VARCHAR(20),
    sex CHAR(1), 
    birth DATE, 
    death DATE
);

6、查看數據表結構

describe  table_name;  --縮寫: desc

7、查看數據表中的記錄

select  *  from  table_name;

-- 去重復
select distinct name from table_name

8、往數據表中添加數據記錄

INSERT INTO table_name
VALUES('puffball','Diane','hanst','f','1999-03-23',NULL);

-- 指定屬性
insert into user3 (name) value('asfjl');

9、刪除數據

delete from table_name where name='puffball';

10、修改數據

update table_name set name='wang' where owner='haha'

11、建表約束--主鍵

create table user(
    id int primary key,
    name varchar(20)
); 

 -- 聯合主鍵
create table user2(
    id int,
    name varchar(20),
    password varchar(20),
    primary key(id,name) 
);

-- 后來添加主鍵
create table user4(
    id int,
    name varchar(20)
);
alter table user4 add primary key(id);

-- 刪除主鍵約束
alter table user4 drop primary key;

-- 修改約束
alter table user4 modify id int primary key;

12、建表約束--自增

create table user3(
    id int primary key auto_increment,
    name varchar(20)
);

12、建表約束--唯一：約束修飾的字段的值不可以重復

create table user5(
    id int,
    name varchar(20),
    unique(name)     
-- 可一起約束多個，不一起重復即可；unique(id,name) 
);

-- 再另一種寫法
create table user5(
    id int,
    name varchar(20)
);
alter table user5 add unique(name);

-- 刪除唯一約束
alter table user5 drop index name

-- modify
alter table user5 modify name varchar(20) unique;

13、非空約束：修飾的字段不能為NULL

create table user6(
    id int,
    name varchar(20) not null 
);
-－ 反null? 異常
insert into user6 (name) value('jfsl');

14、默認約束

create table user7(
    id int,
    name varchar(20),
    age int default 10
);

 insert into user7 (id,name) value(1,'slfj');
 insert into user7 (id,name,age) values(1,'slsfj',5);

15、外鍵約束

create table classes(
    id int primary key,
    name varchar(20)
);
create table students(
    id int primary key,
    class_id int,
    foreign key(class_id) references classes(id)
 );

 

[查詢]

1、多表查詢

-- 兩表查詢
select sname,cno, degree from student,score
where student.sno = score.sno;

-- 三表查詢
select sname, cname,degree from  student,course,course,score
where student.sno = score.sno
and course.cno = score.cno;

2、分組查詢

-- 子查詢加分組求評均
select cno, avg(degree) from score
where sno in (select sno from student where class='1233')
group by cno;

-- year函數與帶in關鍵字的子查詢
select * from student where year(sbirthday) in (select year(sbirthday) from student where sno in (108,117));

3、多層嵌套查詢

-- 
select tno from teacher where tname='zhang';
-- 
select cno from course where tno = (select tno from teacher where tname='zhang');

-- 
select * from score where cno=()
--
select * from score where cno=(select cno from course where tno =(select tno from reacher where tname='zhang'));

4、union與not in

-- not in
select prof from teacher where depart='電子工程系';
select * from teacher where depart='計算機系' and prof not in (select 
prof from teacher where depart='電子工程系');

select * from teacher where depart='電子工程系' and prof not in (select prof from teacher where depart='計算機系');

-- UNION 操作符用於合並兩個或多個 SELECT 語句的結果集
select * from teacher where depart='計算機系' and prof not in (select prof from teacher where depart='電子工程系')
union
select * from teacher where depart='電子工程系' and prof not in (select prof from teacher where depart='計算機系');

5、any與all

-- any表示至少一個
select * from score where cno='34'
and degree>any(select degree from score where cno='334')
order by degree desc;

-- all表示所有
select * from score where cno='34'
and degree>all(select degree from score where cno='334')
order by degree desc;