西安紧急通知南京怎样优化关键词排名
本文主要向大家介绍了MySQL数据库的开发必会的sql语句，通过具体的内容向大家展现，希望对大家学习MySQL数据库有所帮助。
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 t1delete from tb1 where id > 10delete 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.idleft 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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