Given 3 tables:
student(_id, _name, _age, _class)
course(_id, _name)
student_course(_student_id, _course_id, _score)
1. find all students who select a course's number 'c2'.
select _id, _name
from student_course, student
where _course_id = 'c2'
2. find all students who select a course named "Java".
select _id, _name
from student
where _id in (
select _student_id
from student_course sc, course c
where sc._course_id = c._id
and c._name = 'Java'
)
3. find all students who didn't select the course's number 'c5'.
select _id, _name
from student
where _id not in (
select _student_id
from student_course
where _course_id = 'c5'
)
4. find all the student's who selected at least one course.
select count(distinct _student_id)
from student_course
5. find all the student's who selected at least 5 courses.
select _id, _name
from student
where _id in (
select _student_id
from student_course
group by _student_id
having count(_course_id) > 5
)
-- The HAVING clause was added to SQL, because the WHERE keyword could not be used with aggregate functions.
6. find students whose seleted course score all great than 80
select _id, _name
from student
where _id not in (
select distinct _student_id
from student_course
where _score < 80
)
-- the "in/not in" operator will make the inner table left join the outer table.
7. delete redundant student records which are only different in _id.
delete
from student
where _id no in (
select min(_id)
from student
group by _name, _age, _class
)
8. list the student name and the number of their total selected courses.
select _name , count(*)
from (
select _name
from student s
right join student_course sc
on s._id = sc.student_id
)
group by _name
9. How to query table "sale"
year month amount
1991 1 1.1
1991 2 1.2
1991 3 1.3
1991 4 1.4
1992 1 2.1
1992 2 2.2
1992 3 2.3
1992 4 2.4
To the following result:
year m1 m2 m3 m4
1991 1.1 1.2 1.3 1.4
1992 2.1 2.2 2.3 2.4
select
year,
(
select
amount
from sale m
where
m.year = sale.year
and month = 1
) as m1,
(
select
amount
from sale m
where
m.year = sale.year
and month = 2
) as m2,
(
select
amount
from sale m
where
m.year = sale.year
and month = 3
) as m3,
(
select
amount
from sale m
where
m.year = sale.year
and month = 4
) as m4
from sale
group by year
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原文出处：http://lixh1986.iteye.com/blog/2334132
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