select 课程号,课程名称
from course
union
select 课程号,课程名称
from course1

select a.学号,a.姓名,b.课程号
from student as a inner join score as b
on a.学号= b.学号;

select a.学号,a.姓名,b.课程号
from student as a left join score as b
on a.学号=b.学号;

select a.学号,a.姓名,b.课程号
from student as a left join score as b
on a.学号=b.学号
where b.学号 is  not null;

select a.课程号,b.学号,b.姓名
from score as a right join student as b
on a.学号=b.学号;

select a.学号,a.姓名,b.课程号
from student as a right join score as b
on a.学号=b.学号
where b.学号 is  not null;

1）先运行from where group by 和having子句
2）select 子句
3）最后运行order by和limit子句.

select a.学号,a.姓名,count(b.课程号) as选课数,sum(b.成绩) as总成绩
from student as a left join score as b
on a.学号=b.学号
group by a.学号;

select a.学号,a.姓名,avg(b.成绩) as平均成绩
from student as a left join score as b
on a.学号=b.学号
group by a.学号
having avg(b.成绩)>85;

select a.学号,a.姓名,c.课程号,c.课程名称
from student as a inner join score as b on a.学号=b.学号
inner join course as c on b.课程号= c.课程号;

case when <判断表达式> then <表达式>
when <判断表达式> then <表达式>
when <判断表达式> then <表达式>
…
else<表达式>
end

select 学号,课程号,成绩,
(case when 成绩>=60 then'及格'
when 成绩<60 then'及格'
else null
end) as 是否合格
from score;

select 课程号,
sum(case when 成绩 >= 60 then 1
else 0
end) as 及格人数,
sum(case when 成绩<60 then 1
else 0
end) as 不及格人数
from score
group by 课程号;

case when <判断表达式> then <表达式>
when <判断表达式> then <表达式>
when <判断表达式> then <表达式>
…
else<表达式>
end

select a.课程号,b.课程名称,
sum(case when 成绩>85 and 成绩<100 then 1
else 0 end) as '[85-100]',
sum(case when 成绩>70 and 成绩<85 then 1
else 0 end) as '[70-85]',
sum(case when 成绩>60 and 成绩<70 then 1
else 0 end) as '[60-70]',
sum(case when 成绩<60 then 1
else 0 end) as '[<60]'
from score as a right join course1 as b
on a.课程号=b.课程号
group by a.课程号,b.课程名称;