create table class (cid integer primary key , cname text);
create table student (sid integer primary key, sname text);
create table attend ( cid integer , sid integer, grade integer);
Select A.cid, S.sname, S.sid
From Student S, Attend A
Where S.sid=A.sid and not exists
(Select *
From Attend A2
Where A2.cid=A.cid and A2.grade > A.grade);
I don't understand why the result is:
For each class id display the names of students that got the maximum grade in this class
I thought it would instead return the student with minimum grade for each class.
Could anyone help? Thanks!
Note the not operator there - this query select the student(s) that doesn't have any other student with a greater grade than him - i.e., he has the maximum grade.
Related
There are 3 tables student, course, and takes as following
CREATE TABLE student
(
ID varchar(5),
name varchar(20) NOT NULL,
dept_name varchar(20),
tot_cred numeric(3,0) CHECK (tot_cred >= 0),
PRIMARY KEY (ID),
FOREIGN KEY (dept_name) REFERENCES department
ON DELETE SET NULL
)
CREATE TABLE takes
(
ID varchar(5),
course_id varchar(8),
sec_id varchar(8),
semester varchar(6),
year numeric(4,0),
grade varchar(2),
PRIMARY KEY (ID, course_id, sec_id, semester, year),
FOREIGN KEY (course_id, sec_id, semester, year) REFERENCES section
ON DELETE CASCADE,
FOREIGN KEY (ID) REFERENCES student
ON DELETE CASCADE
)
CREATE TABLE course
(
course_id varchar(8),
title varchar(50),
dept_name varchar(20),
credits numeric(2,0) CHECK (credits > 0),
PRIMARY KEY (course_id),
FOREIGN KEY (dept_name) REFERENCES department
ON DELETE SET NULL
)
tot_cred column data in the student table now is assigned with random values (not correct), I want to perform the query that updates and renews those data based on the course's grade each student has taken. For those students who received F grade will be excluded and those who didn't take any course will be assigned 0 as tot_cred.
I came up with two approaches, one is
UPDATE student
SET tot_cred = (SELECT SUM(credits)
FROM takes, course
WHERE takes.course_id = course.course_id
AND student.ID = takes.ID
AND takes.grade <> 'F'
AND takes.grade IS NOT NULL)
This query meets all my needs, but for those students who didn't take any course, it does assign NULL value instead of 0.
The second is using case when
UPDATE student
SET tot_cred = (select sum(credits)
case
when sum(credits) IS NOT NULL then sum(credits)
else 0 end
FROM takes as t, course as c
WHERE t.course_id = c.course_id
AND t.grade<>'F' and t.grade IS NOT NULL
)
But it assigned 0 to all students. Is any way to achieve the above requirement?
If the 1st query meets your requirement and the only problem is that it returns NULL for the students that did not take any course then the easiest solution would be to use instead of SUM() aggregate function the function TOTAL() which will return 0 instead of NULL:
UPDATE student AS s
SET tot_cred = (
SELECT TOTAL(c.credits)
FROM takes t INNER JOIN course c
ON t.course_id = c.course_id
WHERE t.ID = s.ID AND t.grade <> 'F' AND t.grade IS NOT NULL
);
The same could be done with COALESCE():
SELECT COALESCE(SUM(credits), 0)...
Also, use a proper join with an ON clause and aliases for the tables to improve readability.
My problem:
In #4, I'm having trouble joining two Views because the other has an aggregate function. Same with #5
Question:
Create a view name it as studentDetails, that would should show the student name, enrollment date, total price per unit and subject description of students who are enrolled on the subject Science or History.
Create a view, name it as BiggestPrice, that will show the subject id and highest total price per unit of all the subjects. The view should show only the highest total price per unit that are greater than 1000.
--4.) Create a view name it as studentDetails, that would should show the student name,
-- enrollment date the total price per unit and subject description of students who are
-- enrolled on the subject Science or History.
CREATE VIEW StudentDetails AS
SELECT StudName, EnrollmentDate
--5.) Create a view, name it as BiggestPrice, that will show the subject id and highest total
-- price per unit of all the subjects. The view should show only the highest total price per unit
-- that are greater than 1000.
CREATE VIEW BiggestPrice AS
SELECT SubjId, SUM(Max(Priceperunit)) FROM Student, Subject
GROUP BY Priceperunit
Here is my table:
CREATE TABLE Student(
StudentId char(5) not null,
StudName varchar2(50) not null,
Age NUMBER(3,0),
CONSTRAINT Student_StudentId PRIMARY KEY (StudentId)
);
CREATE table Enrollment(
EnrollmentId varchar2(10) not null,
EnrollmentDate date not null,
StudentId char(5) not null,
SubjId Number(5) not null,
constraint Enrollment_EnrollmentId primary key (EnrollmentId),
constraint Enrollment_StudentId_FK foreign key (StudentId) references Student(StudentId),
constraint Enrollment_SubjId_Fk foreign key (SubjId) references Subject(SubjId)
);
Create table Subject(
SubjId number(5,0) not null,
SubjDescription varchar2(200) not null,
Units number(3,0) not null,
Priceperunit number(9,0) not null,
Constraint Subject_SubjId_PK primary key (SubjId)
);
Since this appears to be a homework question.
You need to use JOINs. Your current query:
CREATE VIEW StudentDetails AS
SELECT StudName, EnrollmentDate
Does not have a FROM clause and the query you have for question 5 uses the legacy comma join syntax with no WHERE filter; this is the same as a CROSS JOIN and will connect every student to every subject and is not what you want.
Don't use the legacy comma join syntax and use ANSI joins and explicitly state the join condition.
SELECT <expression list>
FROM student s
INNER JOIN enrollment e ON ...
INNER JOIN subject j ON ...
Then you can fill in the ... based on the relationships between the tables (typically the primary key of one table = the foreign key of another table).
Then for the <expression list> you need to include the columns asked for in the question: student name and enrolment date and subject name would just be those columns from the appropriate tables; and total price-per-unit (which I assume is actually total-price-per-subject) would be a calculation.
Then for the last part of question 4.
who are enrolled on the subject Science or History.
Add a WHERE filter to only include rows for those subjects.
For question 5, you do not need any JOINS as the question only asks about details in the SUBJECT table.
You need to add a WHERE filter to show "only the highest total price per unit that are greater than 1000". This is a simple multiplication and then you can filter by comparing if it is > 1000.
Then you need to limit the query to return only the row with the "highest total price per unit of all the subjects". From Oracle 12, this would be done with an ORDER BY clause in descending order of total price and then using FETCH FIRST ROW ONLY or FETCH FIRST ROW WITH TIES.
Not sure if i get it fully, but i think its this :
Notes:
Always use Id's to filter records:
where su.SubjId in (1,2)
You can find max record using max() at subquery and join it with main query like this :
where su2.SubjId = su.SubjId
You cannot use alias as filter so you can filter it like:
( su.Units * su.Priceperunit ) > 1000
CREATE VIEW StudentDetails AS
select s.StudName,
e.EnrollmentDate,
su.SubjDescription,
su.Units * su.Priceperunit TotalPrice
from student s
inner join Enrollment e
on e.StudentId = s.StudentId
inner join Subject su
on su.SubjId = e.SubjId
where su.SubjId in (1,2)
CREATE VIEW BiggestPrice AS
select su.SubjId, ( su.Units * su.Priceperunit ) TotalPrice
from Subject su
where ( su.Units * su.Priceperunit ) =
(
select max(su2.Units * su2.Priceperunit)
from Subject su2
where su2.SubjId = su.SubjId
)
and ( su.Units * su.Priceperunit ) > 1000
CREATE TABLE Course (
courseno VARCHAR(7),
credits INTEGER NOT NULL,
enroll_limit INTEGER,
classroom VARCHAR(10),
PRIMARY KEY(courseNo), );
CREATE TABLE Student (
sID CHAR(8),
sName VARCHAR(30),
major VARCHAR(10),
trackcode VARCHAR(10),
PRIMARY KEY(sID),
FOREIGN KEY (major,trackcode) REFERENCES Tracks(major,trackcode) );
CREATE TABLE Enroll (
courseno VARCHAR(7),
sID CHAR(8),
grade FLOAT NOT NULL,
PRIMARY KEY (courseNo, sID),
FOREIGN KEY (courseNo) REFERENCES Course(courseNo),
FOREIGN KEY (sID) REFERENCES Student(sID) );
So far I've been able to create two seperate queries, one that counts the number of people who failes. And the other counts the number of people who passed. I'm having trouble combining these to produce the number of people passed / number of people failed. For each course.
SELECT course.courseno, COUNT(*) FROM course inner join enroll on enroll.courseno = course.courseno
WHERE course.courseno LIKE 'CptS%' and enroll.grade < 2
GROUP BY course.courseno;
SELECT course.courseno, COUNT(*) FROM course inner join enroll on enroll.courseno = course.courseno
WHERE course.courseno LIKE 'CptS%' and enroll.grade > 2
GROUP BY course.courseno;
The end result should look something like
courseno passrate
CptS451 100
CptS323 100
CptS423 66
You can do a conditional average for this:
select
courseno,
avg(case when grade > 2 then 100.0 else 0 end) passrate
from enroll
where courseno like 'CptS%'
CREATE TABLE members
(
name varchar(40),
ID char(6) PRIMARY KEY
);
CREATE TABLE games
(
name varchar(100),
ID serial PRIMARY KEY
);
CREATE TABLE ratings
(
memberID char(6) REFERENCES members(ID),
rating SMALLINT CHECK(rating >= 1 AND rating <= 8),
gameID integer REFERENCES games(ID),
PRIMARY KEY (memberID, gameID)
);
I am trying to find all the ratings that were given to the game that has an ID of (2) following by each member who rated it.
I used:
SELECT rating, name
FROM ratings, members
WHERE gameID = 2;
Whenever i used this command, it gives me the correct rating value but it lists all the members even if the member did not rate the game. Can someone help to figure out how to solve the problem.
thanks all in advance
You're looking for JOIN:
SELECT rating, name
FROM ratings r
INNER JOIN members m
ON r.MemberID = m.ID
WHERE gameID = 2;
try:
SELECT rating, name
FROM ratings, members
WHERE gameID = 2 and ratings.memberID = members.id;
I am beginner at SQL and I am trying to create a query.
I have these tables:
CREATE TABLE Hospital (
hid INT PRIMARY KEY,
name VARCHAR(127) UNIQUE,
country VARCHAR(127),
area INT
);
CREATE TABLE Doctor (
ic INT PRIMARY KEY,
name VARCHAR(127),
date_of_birth INT,
);
CREATE TABLE Work (
hid INT,
ic INT,
since INT,
FOREIGN KEY (hid) REFERENCES Hospital (hid),
FOREIGN KEY (ic) REFERENCES Doctor (ic),
PRIMARY KEY (hid,ic)
);
The query is: What is the average in each country of the number of doctors working in hospitals of that country (1st column: each country, 2nd column: average)? Thanks.
You first need to write a query that counts the doctors per hospital
select w.hid, count(w.ic)
from work w
group by w.hid;
Based on that query, you can retrieve the average number of doctors per country:
with doctor_count as (
select w.hid, count(w.ic) as cnt
from work w
group by w.hid
)
select h.country, avg(dc.cnt)
from hospital h
join doctor_count dc on h.hid = dc.hid
group by h.country;
If you have an old DBMS that does not support common table expressions the above can be rewritten as:
select h.country, avg(dc.cnt)
from hospital h
join (
select w.hid, count(w.ic) as cnt
from work
group by w.hid
) dc on h.hid = dc.hid;
Here is an SQLFiddle demo: http://sqlfiddle.com/#!12/9ff79/1
Btw: storing date_of_birth as an integer is a bad choice. You should use a real DATE column.
And work is a reserved word in SQL. You shouldn't use that for a table name.