MySQL必会的SQL语句

本文谈谈MySQL的开发必会的sql语句

创建数据库
create database db1;
删除数据库
drop database db1;
创建数据表
create table tb1用户表(
id int not null auto_increment primary key,
name char(10),
department_id int,
p_id int,
)engine=innodb default charset=utf8;
主键(primary key)一个表只能有一个主键,主键可以由一列或者多列组成

外键的创建
CREATE TABLE t5 (
nid int(11) NOT NULL AUTO_INCREMENT,
pid int(11) not NULL,
num int(11),
primary key(nid,pid) --这里就是把两列设置成了主键
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

                create table t6( 
                    id int auto_increment primary key, 
                    name char(10), 
                    id1 int, 
                    id2 int, 
                    CONSTRAINT fk_t5_t6 foreign key (id1,id2) REFERENCES t1(nid,pid) --这里是设置外键 
                )engine=innodb default charset=utf8; 

数据行的操作
数据的插入
insert into tb1(name,age) values(‘ax’,8);
insert into tb12(name,age) select name,age from tb11;
表中的数据的删除
delete from t1;
truncate table t1;
drop table t1
delete from tb1 where id 10
delete from tb12 where id =2 or name=‘alex’
数据的更新
update tb1 set name=‘root’ where id 10
数据的查询
select * from tb;
select id,name from tb;
表结构的查看
show create table t1;
desc t1;
其他
select * from tb12 where id != 1
select * from tb12 where id in (1,5,12);
select * from tb12 where id not in (1,5,12);
select * from tb12 where id in (select id from tb11)
select * from tb12 where id between 5 and 12;
通配符
select * from tb12 where name like a%
select * from tb12 where name like a_
分页
select * from tb12 limit 10;
select * from tb12 limit 0,10;
select * from tb12 limit 10,10;
select * from tb12 limit 20,10;
select * from tb12 limit 10 offset 20;

page = input(‘请输入要查看的页码’) page = int(page) (page-1) * 10 select * from tb12 limit 0,10; 1 select * from tb12 limit 10,10;2

排序
select * from tb12 order by id desc; 大到小
select * from tb12 order by id asc; 小到大
select * from tb12 order by age desc,id desc;

取后10条数据
select * from tb12 order by id desc limit 10;
分组
select count(id),max(id),part_id from userinfo5 group by part_id;
聚合函数有下面几个:
count
max
min
sum
avg

**** 如果对于聚合函数结果进行二次筛选时?必须使用having ****
select count(id),part_id from userinfo5 group by part_id having count(id) 1;

select count(id),part_id from userinfo5 where id 0 group by part_id having count(id) 1;
自增值设置
表自增值的设置
alter table t1 auto_increment=20;
– 这个就表示从开始20开始算,用上面的show create table t1\G 就可以看到当前的值是多少。
基于会话级别
– 查看当前的会话值
show session variables like ‘auto_incre%’
– 设置会话步长
set session auto_increment_increment=2;
– 设置起始值
set session auto_increment_offset=10;
基于全局设置
– 查看全局的设置值
show global variables like ‘auto_inc%’;
– 设置全局步长值
set global auto_increment_increment=3;
– 设置起始值
set global auto_increment_offset=11;
sql server 是在创建表的时候就可以自己设置,灵活度很高REATE TABLE t5 (nid int(11) NOT NULL AUTO_INCREMENT,pid int(11) NOT NULL,num int(11) DEFAULT NULL,PRIMARY KEY (nid,pid)) ENGINE=InnoDB AUTO_INCREMENT=4, 步长=2 DEFAULT CHARSET=utf8

CREATE TABLE t6 (

nid int(11) NOT NULL AUTO_INCREMENT,pid int(11) NOT NULL,num int(11) DEFAULT NULL,PRIMARY KEY (nid,pid)) ENGINE=InnoDB AUTO_INCREMENT=4, 步长=20 DEFAULT CHARSET=utf8

唯一索引
create table t1(
id int,
num int,
xx int,
unique qu1 (num ,xx) – 意思就是这两列在一行上面数据不能相同,例如都是1,1,就不行
);
唯一索引:约束不能重复(可以为空)主键索引:约束不能重复(不可以为空)他们的特点都是加速查询

外键一对一
create table userinfo1(
id int auto_increment primary key,
name char(10),
gender char(10),
email varchar(64)
)engine=innodb default charset=utf8;

            create table admin( 
                id int not null auto_increment primary key, 
                username varchar(64) not null, 
                password VARCHAR(64) not null, 
                user_id int not null, 
                unique uq_u1 (user_id), 
                CONSTRAINT fk_admin_u1 FOREIGN key (user_id) REFERENCES userinfo1(id) 
            )engine=innodb default charset=utf8; 

外键多对多
示例1:
用户表
相亲表

        示例2: 
            用户表 
            主机表 
            用户主机关系表 
        ===》多对多 

            create table userinfo2( 
                id int auto_increment primary key, 
                name char(10), 
                gender char(10), 
                email varchar(64) 
            )engine=innodb default charset=utf8; 

            create table host( 
                id int auto_increment primary key, 
                hostname char(64) 
            )engine=innodb default charset=utf8; 

            create table user2host( 
                id int auto_increment primary key, 
                userid int not null, 
                hostid int not null, 
                unique uq_user_host (userid,hostid), 
                CONSTRAINT fk_u2h_user FOREIGN key (userid) REFERENCES userinfo2(id), 
                CONSTRAINT fk_u2h_host FOREIGN key (hostid) REFERENCES host(id) 
            )engine=innodb default charset=utf8; 

连表操作
select * from userinfo5,department5

                select * from userinfo5,department5 where userinfo5.part_id = department5.id 

                select * from userinfo5 left join department5 on userinfo5.part_id = department5.id 
                select * from department5 left join userinfo5 on userinfo5.part_id = department5.id 
                # userinfo5左边全部显示 

                # select * from userinfo5 right join department5 on userinfo5.part_id = department5.id 
                # department5右边全部显示 

                select * from userinfo5 innder join department5 on userinfo5.part_id = department5.id 
                将出现null时一行隐藏 

select * from
department5
left join userinfo5 on userinfo5.part_id = department5.id
left join userinfo6 on userinfo5.part_id = department5.id

select
score.sid,
student.sid
from
score

left join student on score.student_id = student.sid

left join course on score.course_id = course.cid

left join class on student.class_id = class.cid

left join teacher on course.teacher_id=teacher.ti

select count(id) from userinfo5;

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