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'
Related
I have three tables tblCourse , tblDegree , tblStudent. I have created a course and degree relation table as like tblCourseDegreeRelation. This relational table uses foreign keys of course and degree table as like:
The tblCourseDegreeRelation table is like:
The tblCourse table is like:
The tblDegree table is like:
The tblStudent (In this table the degree id is foreign key d_id) table is like:
I need to get all records including the tblStudent all record using this query:
SELECT * from tbldegree d
INNER JOIN tblcoursedegreerelation cdr ON d.d_id = cdr.d_id
INNER JOIN tblcourse c ON cdr.c_id = c.c_id
INNER JOIN tblstudent s ON d.d_id = s.d_id
ORDER BY cdr.cdr_id DESC
But this only returns the one student's record while I've two students in the database see below:
How I can get all students records from the joins query?
In your case you have all inner joins, so it will return results where both/all tables satisfies their criteria (on clause).
Viewing your data your student 1 => Ali has a relation with degree 1 =>BS Information Technology.
Further degree 1 has courses (1 => Programming, 2 => English, 5=> Mathematics , 6 => Electronics)
So for student 1 your inner join clause works because it has data in all joined tables.
Now if we look for your student 3 => Bilal who has a relation with degree 3 => BS Mathematics, But this degree has no assigned courses that is why your student Bilal isn't returned
To get all students no matter their related degree has courses you can turn your inner joins into left join not for all tables but for tblcoursedegreerelation and tblcourse
SELECT *
FROM tblstudent s
INNER JOIN tbldegree d ON d.d_id = s.d_id
LEFT JOIN tblcoursedegreerelation cdr ON d.d_id = cdr.d_id
LEFT JOIN tblcourse c ON cdr.c_id = c.c_id
ORDER BY cdr.cdr_id DESC
Demo
In the result set you can see following columns as null due to no association with courses
cdr_id, c_id, d_id, c_id, c_name, c_credit
Just do a Right join on tblstudent:
SELECT * from tbldegree d
INNER JOIN tblcoursedegreerelation cdr ON d.d_id = cdr.d_id
INNER JOIN tblcourse c ON cdr.c_id = c.c_id
RIGHT JOIN tblstudent s ON d.d_id = s.d_id
ORDER BY cdr.cdr_id DESC
EDIT
This way is better:
SELECT c.d_id,c.d_name,c.d_fee,cdr.cdr_id,cdr_c_id,deg.c_name,deg.d_credit,a.s_id,a.s_name
FROM tblstudent a
left join tblDegree c ON a.d_id = c.d_id
left join tblcoursedegreerelation cdr ON cdr.d_id=c.d_id
left join tblcourse deg on deg.c_id=cdr.c_id
ORDER BY cdr.cdr_id DESC
Create two tables:
Course(Course_id(primary key), Course_name)
Student(Roll_no(primary key), Name,Course_id(Foreign key)) and Retrieve the names of all the students who are admitted in the course 'BSC'.
let, the course_id for BSC be 105.
For which the query will be:
SELECT Name FROM Student WHERE Course_id = 105
Can I query for the name of the student without knowing the Course_id (just using the Course_name) ?
You could use an inner join between the tables
SELECT s.Name
FROM Student s
INNER JOIN Course c on c.course_id = s.Course_id
WHERE c.Course_name = 'your_course_name'
or using like
SELECT s.Name
FROM Student s
INNER JOIN Course c on c.course_id = s.Course_id
WHERE c.Course_name like 'your_course_name'
Or use WHERE IN (...)
SELECT
Student.Name
FROM
Student
WHERE
Student.Course_id IN (
SELECT
Course.cource_id
FROM
Course
WHERE
Course.Name = 'BSC'
)
Yes, you can.
SELECT st.Name
FROM Student st
INNER JOIN Course c on c.course_id = st.Course_id
WHERE
c.Course_name = 'Course_Name';
I have a one to many relationship between departments and users.
Database Design:
users has each own department and department has many users. I wanted to select all the department_name but I have a duplicate values of department_name I want to merge it into one so I planned to use UNION how can I implement this using Inner Join? This is my code so far.
SQL
SELECT D.department_name FROM users U
INNER JOIN departments D ON D.id = U.department_id;
Results:
If you want only the distinct department names, you need to group the users into a comma separated values.
select d.department_name, group_concat(u.id) user_id_list
from departments d inner join users u on d.department_id = u.department_id
group by d.department_name
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.
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;