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1,用於設定所select出來的數據是否允許出現重復行（完全相同的數據行）
all：允許出現——默認不寫就是All（允許的）。
distinct：不允許出現——就是所謂的“消除重復行”
2，where:條件
3，group by：分組依據 后面加表的字段名，通常只進行一個字段的分組
mysql表查詢語法形式：select [all | distinct] 字段名或表達式 from 表名 [where] [group by] [having] [order by] [limit];
練習題:共有下面四張表 學生表：student 教師表：teacher 課程表：course 成績表：score
1，查詢Score表中至少有5名學生選修的並以3開頭的課程的平均分數
--操作表score，以cno分組並且cno是以3開頭，取出各個cno的總數，及與cno對應的degree的平均值
select count(cno),avg(degree) from score where cno like '3%' group by cno;
--從虛擬表中找到cno總數大於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,查詢所有學生的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;
--組成新表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;
--組成有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，查詢“95033”班學生的平均分
--從student取sno class,條件是class是95033的
select sno,class from student where class='95033';
--取出score中sno degree
select sno,degree from score;
--將上面兩張表組成一張,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,查詢選修“3-105”課程的成績高於“109”號同學成績的所有同學的記錄
--將109號的成績取出
select degree from score where sno='109' and cno='3-105';
-- 得出最終表
select * from score
where
degree>(select degree from score where sno='109' and cno='3-105')
and cno='3-105';
5,查詢和學號為108的同學同年出生的所有學生的Sno、Sname和Sbirthday列
--找到student中sno為108 的Sbirthday
select year(sbirthday) from student where sno='108';
-- 得出最終表
select sno,sname,sbirthday from student
where
year(sbirthday)=(select year(sbirthday) from student where sno='108');