I need help writing a Sql command. I am pretty new at sql so everything is a little confusing.
The problem:
For each instructor list his / her name and the number of students he / she mentors
There are three tables:
Person with the Names and ID
Instructor with Instructor ID which references ID
Student with StudentID and MentorID which references ID
I tried:
select distinct p.Name,count(d.MentorID)
from Person p, Instructor e, Student d
where e.InstructorID = d.MentorID
and p.ID = e.InstructorID;
But that only gives me one results instead a count per instructor.
Build a group and then count() counts for every group and not the complete result.
select p.Name, count(d.MentorID)
from Person p
join Instructor e on p.ID = e.InstructorID
join Student d on e.InstructorID = d.MentorID
group by p.Name
Related
I have three tables: Students, Friends, and Packages. Students contain two columns: ID and Name. Friends contain two columns: ID and Friend_ID (ID of the ONLY best friend). Packages contain two columns: ID and Salary (offered salary in $ thousands per month)
I have to write a query to output the names of those students whose best friends got offered a higher salary than them. Names must be ordered by the salary amount offered to the best friends. It is guaranteed that no two students got the same salary offer.
Here is my tables:
Student Table Friends Table Packges Table
And the output should like this:Output Table
First join all 3 table with their id while you will get every name with their friend id. Now you have join that friend id with packages id while you will get friends salary. finally filter FRIEND_SALARY>SALARY .
I use oracle syntax here. hope will work fine in mysql also.
select TMP2.NAME name
from (SELECT TMP1.NAME,
PP.ID,
TMP1.SALARY salary,
TMP1.FRIEND_ID,
PP.SALARY friend_salary
FROM packages pp,
(SELECT *
FROM students s, friends f, packages p
WHERE S.ID = F.ID
AND S.ID = P.ID) tmp1
WHERE PP.ID = TMP1.FRIEND_ID) tmp2
where TMP2.FRIEND_SALARY>TMP2.SALARY
I'm fairly new to MySQL, and trying to understand the many-to-many relationship since these examples can popup in interviews
There are 3 tables, and since a Student can have many courses and a Course can have many students, this is a Many-to-Many relationship right?
The tables are
Student- has student ID, name, date of birth, and department.
Courses- Has ID, Name of course
Student_Courses- Has student_id, course_id
How would I display these 2 questions-
1) Given a studentID, return all the names of the courses the student is taking
2) Return the name of students who is taking X amount of courses or more (Ex. 4 or more courses).
Im trying to write queries on these, but I'm stuck...
In the case of selecting all of the courses for a given student ID you could try the following, which will return one row for each Course a Student is associated with.
select
s.name as StudentName,
c.name as CourseName
from `Student` as s
inner join `Student_Course` as sc on (sc.student_id = s.ID)
inner join `Course` as c on (c.ID = sc.course_id)
where
(s.`ID` = 'given_Student_ID_here')
;
As for selecting a list of the names of Students taking N or more courses, for this you might use an aggregating sub-select as a WHERE clause in which we reference one of the outer tables (i.e. [Student]) so that the result of the aggregation is personalised per Student record:
select
s.name as StudentName
from `Student` as s
where
(
(
select count(*)
from `Student_Course` as sc
inner join `Course` as c on (c.ID = sc.course_id)
where (sc.student_id = s.ID)
) >= 4
)
;
You might also consider an alternative approach to this second problem by using the GROUP BY and HAVING clauses:
select
s.name as StudentName
from `Student` as s
inner join `Student_Course` as sc on (sc.student_id = s.ID)
inner join `Course` as c on (c.ID = sc.course_id)
group by
s.name
having
count(*) >= 4
;
Table instructor: ID, name, dept_name, salary
Table student: ID, name, dept_name, tot_cred
Table advisor: s_ID, i_ID which contains student id and instructor id for the two table.
I need to find all the instructor and student's name where the department of the advisor is CComp.Sci
I can find all the id of the instructor and student whrer the intructor is fom Computer science. And only the name of the students.
But can't figure out both the name at the same time.
I wrote this:
SELECT student.name
FROM student
WHERE student.ID in (SELECT advisor.s_ID
FROM advisor
, instructor
WHERE advisor.i_ID = instructor.ID
and instructor.dept_name = 'Comp.Sci')
The root solution I think you need here is just a simple join between the three tables. But since you need a single list of both student and instructor names, this complicates things. One option is to union together a query which finds the matching students along with a query which finds the matching instructors.
SELECT s.name, 'student' AS type
FROM student s
INNER JOIN advisor a
ON s.ID = a.s_ID
INNER JOIN instructor i
ON a.i_ID = i.ID
WHERE i.dept_name = 'CComp.Sci'
UNION ALL
SELECT DISTINCT i.name, 'instructor'
FROM student s
INNER JOIN advisor a
ON s.ID = a.s_ID
INNER JOIN instructor i
ON a.i_ID = i.ID
WHERE i.dept_name = 'CComp.Sci'
Using "where .. in (..)" will give you bad performance, also it will not allow you to get the data from the table in the where clause.
Here is a solution if you mean to get the result of instructor name side alone with student name.
SELECT S.name AS Student,I.name AS Instructor
FROM Students AS S
JOIN Advisor AS A ON A.s_ID = S.Id
JOIN Instructor AS I ON I.Id = A.i_ID
WHERE I.dept_name = 'Comp.Sci'
*notice that I used alias
** if you want for example to get all students even those who lack instructor use LEFT JOIN
How would I list the students who started later than start year of student with id = 8871?
This is what I have so far:
SELECT sid s1
FROM Student
WHERE s1.started <= sid = '8871';
I went to this link for some reference but I'm still having issues:
Query comparing dates in SQL
Student Table has:
Student (Lastname, Firstname, SID, Started)
any help would be great
I'd use a self join where one side of the join contains the students you want and the the reference student 8871:
SELECT a.*
FROM student a
JOIN student b ON b.sid = '8871' AND a.started > b.started
So I'm learning MySQL and I'm trying to do the following:
For each instructor list his / her name and the number of students he / she mentors.
The relevant part of the schema is:
Person(ID, Name)
Student(StudentID, MentorID)
Instructor(InstructorID)
Both InstructorID and StudentID map to Person.ID, and MentorID maps to InstructorID (Each student has an instructor mentor, and both instructors and students are Persons).
I've tried the following to no avail:
select p.Name, count(select s.StudentID
from Student s
where s.MentorID = i.InstructorID)
from Person p, Instructor i
where p.ID = i.InstructorID;
Also this after reading some things on StackOverflow:
select InstructorDetails.Name, count(Mentees)
from Instructor i
inner join Person as InstructorDetails
on InstructorDetails.ID = i.InstructorID
inner join Student as Mentees
on Mentees.MentorID = i.InstructorID;
Any suggestions?
You lack GROUP BY on your query,
SELECT InstructorDetails.Name, count(*) totalCount
FROM Instructor i
INNER JOIN Person as InstructorDetails
ON InstructorDetails.ID = i.InstructorID
INNER JOIN Student as Mentees
ON Mentees.MentorID = i.InstructorID
GROUP BY InstructorDetails.Name