I have the following database schema:
table courses:
id
tutor_id
title
table course_categories:
id
category_id
course_id
table categories:
id
name
table tutors:
id
name
table subscribers:
id
course_id
user_id
I need to make 1 sql to get a course with all it's categories, and the tutor for that course and the number of subscribers for that course. Can this be done in 1 query? Should this be done using stored procedures?
With this query you get what you want:
select co.title as course,
ca.name as category,
t.name as tutor,
count(s.*) as total_subscribers
from courses co
inner join course_categories cc on c.id = cc.course_id
inner join categories ca on cc.category_id = ca.id
inner join tutors t on co.tutor_id = t.tutor_id
left join subscribers s on co.id = s.course_id
where co.title = 'Cat1'
group by co.title, ca.name, t.name
I used left join on subscribers because there might be no one for a given course. I'm assuming that all the other tables have data on it for every course, categorie and tutor. If not, you can user left join as well but then you'll have data with null.
It can be done. You need to look up select and the use of join. See select and join to help complete the assignment
select cou.title, cat.name, tu.name, count(sub.user_id) from courses cou, course_categories cca, categories cat, tutors tu, subscribers sub where cou.id = cca.id and cat.id = tu.id and tu.id = sub.id group by cou.title, tu.name;
Related
I have five tables: 1. courses 2.departmetns 3.students 4. enroll_courses 5.results
Departments has id
Courses has id, department_id
Students has id,department_id
Enroll_courses has id, courses_id,students_id
Results has id, courses_id,students_id
I have to select courses.name which are in enroll_courses tables but not in results table.
I have written the query, I couldn't get the exact answer. If anyone can help me to find the solution.
Query:
SELECT courses.name
FROM courses
JOIN departments ON courses.department_id = departments.id
JOIN students ON departments.id = students.department_id
WHERE students.id = 9
AND courses.id IN (SELECT course_id FROM enroll_courses)
Its a little bit blind shoot since you didn't provide any DDL's , but I assume results table have a course_id column , if not, change it to the relation. You can do this with EXISTS() and NOT EXISTS():
SELECT courses.name
FROM courses
JOIN departments ON courses.department_id = departments.id
JOIN students ON departments.id = students.department_id
WHERE EXISTS(SELECT 1 FROM enroll_courses ec WHERE ec.course_id = courses.id)
AND NOT EXISTS(SELECT 1 FROM results r WHERE r.course_id = courses.id)
AND students.id =9
I want to do a relational division query. First, here is the structure of each table:
Student
id
name
Course
id
name
Student_passed_course (junction table that stores who passed which course)
id_student
id_course
Basically, what I want is to get the names of the students that have passed all the courses that exist in table Course using JOIN (or LEFT JOIN, etc). I already implemented a solution using NOT EXISTS.
Also this is the equation I made.
A solution based on JOIN could be as follow:
SELECT s.name AS student_name, c.name as course_name FROM (
SELECT s_id, c_id, SUM(c_exists) AS c_exists, SUM(c_taken) as c_taken, SUM(c_exists)-SUM(c_taken) as not_taken FROM (
SELECT s.id as s_id, c.id as c_id, 1 as c_taken, 0 as c_exists FROM student s JOIN student_passed_course spc ON spc.id_student = s.id JOIN course c ON c.id = spc.id_course
UNION ALL
SELECT s.id as s_id, c.id as c_id, 0 as c_taken, 1 as c_exists FROM student s JOIN course c
) X group BY s_id, c_id) Y
JOIN student s ON s.id = s_id JOIN course c ON c_id = c.id
WHERE not_taken = 1;
I don't know if this would be more or less efficient than your solution.
There are three tables
Students, Courses and Registration
Students has id, name columns
Courses has also course.id, course.name
and there is third table joining the Students and Courses table
Registration : stu_id, course_id
One Student can take one or many courses.
I would like to find the name of Students registered in only one course.
Try with INNER JOIN
SELECT S.id, S.name
FROM students S
INNER JOIN registration R ON S.id = R.stu_id
GROUP BY S.id, S.name
HAVING COUNT(*) = 1
Like below:
SELECT s.id, s.name
FROM students s
LEFT JOIN registration r ON s.id = r.stu_id
GROUP BY s.id, s.name
HAVING COUNT(r.course_id) = 1
select s.*
from (
select r.stu_id stu_id
from Registration r
group by r.stu_id
having count(*) == 1) ra
join Students s on s.id = ra.stu_id;
This one is more efficient.
It's unlikely that your schema has null fields. Therefore, it doesn't matter which kind of join, inner or left, you use.
I have a MySql query problem. The tables I'm using are very large, so I listed a simple example that I can then use in my more complex table structure.
Lets say the tables are the following.
House(id, name)
Person(id, name, house_id)
Car(id, name, person_id, type)
CarEngine(id, name, hp)
Each Person belongs to a House. Each Car belongs to a Person. Each Car has a CarEngine with the same primary key (id).
How would I select only the name from each table while selection all the information from CarEngine from these tables efficiently where Car type is truck?
select p.name as PersonNamee, h.name as HouseName, c.name as CarName,
e.id as CarID, e.name as EngineName, e.hp
from Person p
inner join House h on p.house_id = h.id
inner join Car c on p.id = c.person_id
inner join CarEngine e on c.id = e.id
where c.type = 'truck'
I have a users table which contains the users information (fname, lname...etc) and a invoices table. In the invoices table I have a field called created_id which links to the user.id that created the invoice. I also have a field called staff_id which links to the staff user.id that approved the invoice.
How can I query the first and last name for both the created_id and the staff_id in a single query? Here are a few things I've tried....
SELECT
invoices.*,
users.fname as created_fname,
users.lname as created_lname
FROM
invoices
INNER JOIN users
ON users.id = invoices.created_id;
This works, but it only gets me the person's name that created the invoice. How can I add the staff's name to that as well....
SELECT
invoices.*,
users.fname as created_fname,
users.lname as created_lname,
users2.fname as staff_fname,
users2.lname as staff_lname
FROM invoices, users
LEFT JOIN
invoices,
users AS users2
ON
users.id = invoices.created_id,
users.id = users2.id
That doesn't work, but is closer. Any guidance or examples would be very helpful. Also, if you have any recommendations for good books on learning how to do more advanced MySQL queries that would be helpful too.
You need to join users table twice on table Invoice.
SELECT a.*,
b.fname created_firstName,
b.lname created_LastName,
c.fname staff_firstName,
c.lname staff_LastName
FROM Invoice a
INNER JOIN users b
ON a.created_id = b.id
INNER JOIN users c
ON a.staff_id = c.id
and best thing is you can concatenate their names into one using CONCAT
SELECT a.*,
CONCAT(b.fname, ' ', b.lname) created_fullName,
CONCAT(c.fname, ' ', c.lname) staff_fullName
FROM Invoice a
INNER JOIN users b
ON a.created_id = b.id
INNER JOIN users c
ON a.staff_id = c.id