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1,用于设定所select出来的数据是否允许出现重复行（完全相同的数据行）
all：允许出现——默认不写就是All（允许的）。
distinct：不允许出现——就是所谓的“消除重复行”
2，where:条件
3，group by：分组依据 后面加表的字段名，通常只进行一个字段的分组
mysql表查询语法形式：select [all | distinct] 字段名或表达式 from 表名 [where] [group by] [having] [order by] [limit];
练习题:共有下面四张表 学生表：student 教师表：teacher 课程表：course 成绩表：score
1，查询Score表中至少有5名学生选修的并以3开头的课程的平均分数
--操作表score，以cno分组并且cno是以3开头，取出各个cno的总数，及与cno对应的degree的平均值
select count(cno),avg(degree) from score where cno like '3%' group by cno;
--从上面的虚拟表中找到cno总数大于5的
select * from
(select count(cno) a,avg(degree) b from score where cno like '3%' group by cno) c
where a>5;
2,查询所有学生的Sno、Cname和Degree列
--找到student的sno
select sno from student;
--找到score 的sno,degree,cno
select sno,degree,cno from score;
--找到course的cno
select cno,cname from course;
--组成新表cno sno degree
select a.cno,b.sno,b.sname,a.degree from (select sno,degree,cno from score) a join (select sno,sname from student) b on a.sno=b.sno;
--组成有cname cno sn degree sname的表
select d.cname,e.cno,e.sno,e.degree,e.sname
from
(select a.cno,b.sno,b.sname,a.degree from (select sno,degree,cno from score) a join (select sno,sname from student) b on a.sno=b.sno) as e
join
(select cname,cno from course) as d
on d.cno=e.cno;
3，查询“95033”班学生的平均分
--从student取sno class,条件是class是95033的
select sno,class from student where class='95033';
--取出score中sno degree
select sno,degree from score;
--将上面两张表组成一张,取degree的平均值
select avg(degree) from
(select sno,class from student where class='95033') a
join
(select sno,degree from score) b
on a.sno=b.sno;
4,查询选修“3-105”课程的成绩高于“109”号同学成绩的所有同学的记录
--将109号的成绩取出
select degree from score where sno='109' and cno='3-105';
-- 得出最终表
select * from score
where
degree>(select degree from score where sno='109' and cno='3-105')
and cno='3-105';
5,查询和学号为108的同学同年出生的所有学生的Sno、Sname和Sbirthday列
--找到student中sno为108 的Sbirthday
select year(sbirthday) from student where sno='108';
-- 得出最终表
select sno,sname,sbirthday from student
where
year(sbirthday)=(select year(sbirthday) from student where sno='108');
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