mysql 基本操作（三）

show tables;
desc book;
select * from book;
alter table book add sscore tinyint unsigned default 60;
insert into book values (null,'李五四','98'),(null,'王','10'),(null,'王三五','20');
insert into book values (null,'李','98'),(null,'李二','10'),(null,'李三','20');
/*查看book表格里面的指定的列，顺序可以随意改变*/
select    bookid,bookname from book;
/*设定book表里面列的别名设定*/
select bookname 姓名,sscore 成绩 from book;
/*按照sscore 升序*/
select * from book order by  sscore desc;
/*按照sscore 降序*/
select * from book order by sscore asc;
/*查询book表，姓名为li并且成绩为110的，and为逻辑与，or逻辑或，not逻辑非*/
select * from book where bookname='li' and sscore='110';
/*查询book表里面成绩不等于110的显示出来，<>  !=  都为不等于*/
select * from book where sscore <>110;
select *from book where sscore !=100;
/*说反话，即为成绩为50的，这样效率低尽量不要这样写*/
select * from book where not sscore !=50;
/* in        not in   的使用*/
/*查询book表格里面的bookname列里面的名字为王五，李二的人，or逻辑或*/
select *from book where bookname in('王五','李二');
select * from book where bookname ='李三' or bookname='李二';
/*查询book表格里面的bookname列里面的名字不为王五的人*/
select *from book where bookname not in('王五');
/*删除book表里面id为2,16,17的数据信息*/
delete from book where bookid in (2,16,17);
/*between   and  not between  可以在日期时间数字里面引用*/
select * from book where sscore between 80 and 100 ;
/*查询出分数不在80-100之间数字*/
select * from book where sscore not between 80 and 100;
/*like  not  like  模糊查询  like和=相同  not like 是取反*/
select * from book where bookname like '王五';
select * from book where bookname = '李三';
-- %一定是配合like使用  %表示0个或者多个任意字符，查找表中李开头的
select * from book where bookname like '李%';
-- 检索表格里面包含l的所有的信息
select * from book where bookname like '%l%';
-- -表示单个任意字符,会输出含有2个字的人的信息
select * from book where bookname like '__';
-- 查找李开头的有2个字的
select *from book where bookname like '李_';
-- 正则表达式一个点代表一个字母，3个点相当于一个汉字，一个点表示一个字节，表示一个任意符号 
select * from book where bookname regexp '^......$';
-- 此种格式是不正确的%必须配合like使用如下的写法是没有结果的  
select * from book where bookname ='李%';
-- is null   is not null 统计为空的值  让bookid=21 的成绩修改为0 ’'此为空字符串不是null
update book set sscore =null  where bookid in (21);
select * from book ;
select * from book where sscore is null ;
-- select * from book where sscore = null ;不能这样写是错误的
select * from book where sscore is not null;
-- null 值不参与求平均值，值为0 要参与求平均值
-- 查询总归有多少条数据总和
select count(*) from book;
--  查询book为空的总共有多少条
select count(*) from book where sscore is null ;
-- 查询所有学生的平均成绩  avg表示平均分
select avg (sscore) from book;
-- 查找平均分，最高分，最低分
select avg(sscore) 平均分 ,max(sscore) 最高分 ,min(sscore) 最低分 from book;
-- 分组统计配合使用
select bookname 姓名 count(*) from book group by bookname;
-- 统计出性别为女生的总人数
select count(*) from book  where sgender ='女';
-- 统计出矿考的学生  此处一定不能用=
select count(*) from book where sscore is null;
-- 统计各专业的人数
select 姓名 from book group by bookid;
-- 消除重复的bookid distinct消除
select distinct bookid from book;
use xx;
alter table book add sgender enum('男','女','保密');
insert into book values(50,'小米',90,'女'),(51,'大米',20,'女');
desc book;
select * from book;
drop table student,teacher;
-- 统计各专业人数 group by 根据专业分组 order by avg 根据什么降序排列
-- 平均分排行前两位的
select sdept 专业,count(*) 人数,
avg(sscore) 平均分,
max(sscore) 最高分,
min(sscore) 最低分
from student where sgender='男'
group by sdept  order by avg(sscore) desc limit 2;
having  avg(sscore)<60;
select * from student;
select count(*) from student 
-- 消除重复列的数据 distinct
select distinct sdept from student;
-- max为集合函数不能直接在where中使用所以如下的语句是错误的
select * from student where sscore=max(sscore);错误
-- 查询最高分的学生信息,union 并集查询      子查询
select * from student where sscore in (select max(sscore) from student)
union
select * from student where sscore in (select min(sscore) from student);
-- 连接查询，效率低显示学生学号，姓名成绩，代课老师姓名
select sid,sname,sscore,tname from student ,teacher where stid=tid;
-- 连左查询
select sid,sname,sscore,tname from student  left join teacher on stid=tid;
-- 显示老师 ID学生姓名成绩
select  tid,tname,sid,sname ,sscore from teacher  left join student on tid=stid
order by tname desc;
select * from teacher;
insert into teacher values( null,'王老师');
/*老师名字按照升序排列*/
select * from teacher order by convert (tname using gbk) asc;