SQL : Join of three tables - sql

Question

I am new to SQL and am learning joins of tables now.
I am stuck at joining three tables.
(I have given the rows I have inserted to the tables also for your reference.)
My tables are
--Table1
create table sql_students(
stu_studentid int not null primary key,
stu_name varchar(100) not null,
stu_regnnumber bigint unique not null
)
--Rows inserted to Table1
insert into sql_students (stu_studentid,stu_name,stu_regnnumber) values (1,'John',194300)
insert into sql_students (stu_studentid,stu_name,stu_regnnumber) values (2,'Joy',959595)
insert into sql_students (stu_studentid,stu_name,stu_regnnumber) values (3,'Lucy',474848)
--Table2
create table sql_exam(
exa_examid bigint not null primary key,
exa_name varchar(100) not null,
exa_maxmark decimal(5,2) not null,
exa_minmarkreqdforpass decimal(5,2) not null,
exa_examscheduletime datetime not null
)
--Rows inserted into Table2
insert into sql_exam(exa_examid,exa_name,exa_maxmark,exa_minmarkreqdforpass,exa_examscheduletime) values (1,'Maths',100,40,'2012-10-10 10:00')
insert into sql_exam(exa_examid,exa_name,exa_maxmark,exa_minmarkreqdforpass,exa_examscheduletime) values (2,'English',75,35,'2012-10-11 10:00')
--Table3
create table sql_studentmarks(
stm_studentid int foreign key references sql_students(stu_studentid),
stm_examid bigint foreign key references sql_exam(exa_examid),
stm_mark decimal(5,2)
)
--Rows inserted into Table3
insert into sql_studentmarks(stm_studentid,stm_examid,stm_mark) values (1,1,80)
insert into sql_studentmarks(stm_studentid,stm_examid,stm_mark) values (2,1,90)
insert into sql_studentmarks(stm_studentid,stm_examid,stm_mark) values (3,1,40)
insert into sql_studentmarks(stm_studentid,stm_examid,stm_mark) values (1,2,70)
insert into sql_studentmarks(stm_studentid,stm_examid,stm_mark) values (2,2,60)
insert into sql_studentmarks(stm_studentid,stm_examid,stm_mark) values (3,2,17)
I need guidelines to get
All students who passed all exams
All students who attended all exams
All students and their mark difference in Maths and English.
Thanks in advance.

Answer 1

For #1
SELECT s.stu_name
FROM sql_studentmarks AS m
JOIN sql_students AS s ON m.stm_studentid = s.student_id
JOIN sql_exam AS x ON COUNT(m.stm_examid) = COUNT (x.stm_examid)
WHERE m.stm_mark >= x.exa_minmarkreqdforpass GROUP BY s.stu_studentid;
For #2 you can base on the exam count, perhaps creating a var in the select statement:
SELECT s.stu_name
FROM sql_studentmarks AS m
JOIN sql_students AS s ON m.stm_studentid = s.stu_studentid
RIGHT OUTER JOIN sql_exam AS x ON COUNT(m.stm_examid) = COUNT (x.stm_examid)
AND m.stm_examid IS NOT NULL
GROUP BY s.stu_student_id;
FOR #3 use the examples from above, simple joins will do:
SELECT s.stu_name, x.exa_name, m.stm_mark
FROM sql_studentmarks AS m
JOIN sql_students AS s ON m.stm_studentid = s.stu_studentid
JOIN sql_exam x ON m.stm_examid = x.exa_examid AND s.stu_studentid = x.exa_studentid;

Answer 2

The code below shows you the first two queries. Your joins are OK.. Try these in SQL Fiddle and then see if you can work the third query out
Query #1: (You were very close, just needed the distinct keyword)
select stm_studentid,stu_name
from sql_exam
join sql_studentmarks on exa_examid=stm_examid
and stm_mark>exa_minmarkreqdforpass
inner join sql_students on stu_studentid=stm_studentid
group by stm_studentid,stu_name
having count(*) = (select count(*) from sql_exam)
Query #2: -
The group by lets you count the number of exams taken.
The having lets you compare the count to number of exams available
select stu_name,count(*) as NumExamsTaken
from sql_studentmarks
join sql_students on stu_studentid=stm_studentid
group by stu_name
having count(*) = (select count(*) from sql_exam)
Query #3:
This should get you started
select stu_studentid,stu_name,
MG.stm_mark as MathGrade,
EG.Stm_mark as EnglishGrade
from sql_students
join sql_exam MATH on MATH.exa_name='MATHS'
join sql_exam ENG on ENG.exa_name='ENGLISH'
join sql_studentmarks MG on MG.stm_studentid=stu_studentid
and MG.stm_examid=MATH.exa_examId
join sql_studentmarks EG on EG.stm_studentid=stu_studentid
and EG.stm_examid=ENG.exa_examId