子查询

where(这个值是计算出来的)
先执行where里面的语句，之后执行外面的语句

本质：where里面嵌套一个子查询语句

查询参加高等数学考试的学生（学号，姓名。科目编号，成绩）,降序排列

-- 方式一：连接查询

SELECT s.studentno,studentname,r.subjectno,studentresult
FROM student s
INNER JOIN result r
on s.studentno=r.studentno
INNER JOIN `subject`sub
on sub.subjectno=r.subjectno
WHERE subjectname='高等数学-1'
order by studentresult DESC

-- 方式二：子查询

SELECT s.studentno,studentname,r.subjectno,studentresult
FROM student s
INNER JOIN result r
on s.studentno=r.studentno
WHERE subjectno=(
SELECT subjectno from `subject` WHERE subjectname='高等数学-1'
)
order by studentresult DESC

-- 查询分数不小于80的学生的学号，姓名

SELECT s.studentno,studentname
from student s
INNER JOIN result r
on s.studentno=r.studentno
WHERE studentresult>=80

-- 查询参加高等数学-1分数不小于80的学生的学号，姓名

SELECT s.studentno,studentname
from student s
INNER JOIN result r
on s.studentno=r.studentno
WHERE studentresult>=80
 and 
subjectno=(
SELECT subjectno from `subject` WHERE subjectname='高等数学-1'
)

-- 查询参加高等数学-1分数不小于80的学生的学号，姓名
-- 嵌套查询(由里及外)

SELECT studentno,studentname
from student
WHERE studentno in(
SELECT studentno FROM result WHERE studentresult>=80
 and 
subjectno=(
SELECT subjectno from `subject` WHERE subjectname='高等数学-1'
))

-- 查询参加高等数学-1，前5名同学的信息（学号，姓名，成绩）
-- 联表查询

SELECT s.studentno,studentname,studentresult
from student s
INNER JOIN result r
on s.studentno=r.studentno
INNER JOIN `subject` sub
ON r.subjectno=sub.subjectno
WHERE subjectname='高等数学-1'
ORDER BY studentresult DESC
LIMIT 0,5

-- 查询参加高等数学-1，前5名同学的信息（学号，姓名，成绩）
-- 子查询

SELECT s.studentno,studentname,studentresult
from student s
INNER JOIN result r
on s.studentno=r.studentno
WHERE subjectno= (
SELECT subjectno from `subject` WHERE subjectname='高等数学-1'
)
ORDER BY studentresult DESC
LIMIT 0,5