学习笔记：MySQL常用的一些SQL语句

本文谈谈MySQL的开发必会的sql语句

创建数据库

createdatabasedb1;

删除数据库

dropdatabasedb1;

创建数据表

createtabletb1用户表(idintnotnullauto_increment primarykey,namechar(10),                     department_idint,                     p_idint,                 )engine=innodbdefaultcharset=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

CREATETABLEt5 (                       nidint(11)NOTNULLAUTO_INCREMENT,                       pidint(11)notNULL,numint(11),                       primarykey(nid,pid)--这里就是把两列设置成了主键 )ENGINE=InnoDBDEFAULTCHARSET=utf8;createtablet6(idintauto_increment primarykey,namechar(10),                         id1int,                         id2int,CONSTRAINTfk_t5_t6 foreignkey(id1,id2)REFERENCESt1(nid,pid)--这里是设置外键 )engine=innodbdefaultcharset=utf8;

数据行的操作

数据的插入

insertintotb1(name,age)values('ax',8);insertintotb12(name,age)selectname,agefromtb11;

表中的数据的删除

deletefromt1;truncatetablet1;droptablet1deletefromtb1whereid>10deletefromtb12whereid>=2orname='alex'

数据的更新

updatetb1setname='root'whereid>10

数据的查询

select*fromtb;selectid,namefromtb;

表结构的查看

showcreatetablet1; desc t1;

其他

select*fromtb12whereid!=1select*fromtb12whereidin(1,5,12);select*fromtb12whereidnotin(1,5,12);select*fromtb12whereidin(selectidfromtb11)select*fromtb12whereidbetween5and12;

通配符

select*fromtb12wherenamelike"a%"select*fromtb12wherenamelike"a_"

分页

select*fromtb12 limit10;select*fromtb12 limit0,10;select*fromtb12 limit10,10;select*fromtb12 limit20,10;select*fromtb12 limit10offset20;# 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*fromtb12orderbyiddesc; 大到小select*fromtb12orderbyidasc;  小到大select*fromtb12orderbyagedesc,iddesc;  取后10条数据select*fromtb12orderbyiddesclimit10;

分组

selectcount(id),max(id),part_idfromuserinfo5groupbypart_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 ****selectcount(id),part_idfromuserinfo5groupbypart_id havingcount(id) > 1;selectcount(id),part_idfromuserinfo5whereid > 0groupbypart_id havingcount(id) > 1;

自增值设置

表自增值的设置

altertablet1 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 showsessionvariableslike'auto_incre%'-- 设置会话步长 setsessionauto_increment_increment=2;-- 设置起始值 setsessionauto_increment_offset=10;

基于全局设置

-- 查看全局的设置值 showglobalvariableslike'auto_inc%';-- 设置全局步长值 setglobalauto_increment_increment=3;-- 设置起始值 setglobalauto_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

createtablet1(idint,numint,     xxint,uniquequ1 (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

createtableuserinfo1(idintauto_increment primarykey,namechar(10),                     genderchar(10),                     emailvarchar(64)                 )engine=innodbdefaultcharset=utf8;createtableadmin(idintnotnullauto_increment primarykey,                     usernamevarchar(64)notnull,passwordVARCHAR(64)notnull,                     user_idintnotnull,uniqueuq_u1 (user_id),CONSTRAINTfk_admin_u1 FOREIGNkey(user_id)REFERENCESuserinfo1(id)                 )engine=innodbdefaultcharset=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createtableuserinfo2(idintauto_increment primarykey,namechar(10),                     genderchar(10),                     emailvarchar(64)                 )engine=innodbdefaultcharset=utf8;createtablehost(idintauto_increment primarykey,                     hostnamechar(64)                 )engine=innodbdefaultcharset=utf8;createtableuser2host(idintauto_increment primarykey,                     useridintnotnull,                     hostidintnotnull,uniqueuq_user_host (userid,hostid),CONSTRAINTfk_u2h_user FOREIGNkey(userid)REFERENCESuserinfo2(id),CONSTRAINTfk_u2h_host FOREIGNkey(hostid)REFERENCEShost(id)                 )engine=innodbdefaultcharset=utf8;

连表操作

select*fromuserinfo5,department5select*fromuserinfo5,department5whereuserinfo5.part_id = department5.idselect*fromuserinfo5leftjoindepartment5onuserinfo5.part_id = department5.idselect*fromdepartment5leftjoinuserinfo5onuserinfo5.part_id = department5.id                     # userinfo5左边全部显示                      #select*fromuserinfo5rightjoindepartment5onuserinfo5.part_id = department5.id                     # department5右边全部显示select*fromuserinfo5 innderjoindepartment5onuserinfo5.part_id = department5.id                     将出现null时一行隐藏select*fromdepartment5leftjoinuserinfo5onuserinfo5.part_id = department5.idleftjoinuserinfo6onuserinfo5.part_id = department5.idselectscore.sid,                         student.sidfromscoreleftjoinstudentonscore.student_id = student.sidleftjoincourseonscore.course_id = course.cidleftjoinclassonstudent.class_id = class.cidleftjointeacheroncourse.teacher_id=teacher.tiselectcount(id)fromuserinfo5;