My question is about a SQL query that returns a course list (see image).
Instead of the class_id, I would like to print the name of the class. The class name is stored in table classes. I already tried it with INNER JOIN, but unfortunately I haven't found a solution yet.
select
group_concat(ids) as ids,
group_concat(class_id) as class_ids,
teacher, name, weekday_hours
from
(select
l.class_id, l.teacher_id, l.name,
group_concat(l.weekday, ':', l.hour order by l.weekday, l.hour) as weekday_hours,
group_concat(l.id order by l.id) as ids
from
lessons l
group by
l.class_id, l.teacher_id, l.name) l
group by
teacher, name, weekday_hours;
You want to join in the subquery and then process the name just as you would the class_id:
select group_concat(ids) as ids,
group_concat(class_name) as class_names,
teacher, name, weekday_hours
from (select l.class_id, c.name as class_name,
l.teacher_id, l.name,
group_concat(l.weekday, ':', l.hour order by l.weekday, l.hour) as weekday_hours,
group_concat(l.id order by l.id) as ids
from lessons l join
classes c
on l.class_id = c.id
group by l.class_id, c.name, l.teacher_id, l.name
) l
group by teacher, name, weekday_hours;
Related
I have the following tables:
Students (id, name, surname, study_year, department_id)
Courses(id, name)
Course_Signup(id, student_id, course_id, year)
I want to display the courses to which at least 10 students have signed up for, only using subqueries (no group-by, join or set operations). This could be easily implemented using data aggregation and join:
SELECT c.name, COUNT(csn.course_id)
FROM Course_Signup csn
JOIN Courses c
ON csn.course_id = c.id
GROUP BY c.name
HAVING COUNT(csn.course_id) >= 10
But how would I do this only using subqueries? Is there any other way, other than COUNT, to get the number of courses? Thank you, in advance!
You can use a correlated sub-query to retrieve the name:
SELECT (SELECT c.name FROM Courses c WHERE csn.course_id = c.id) AS name,
COUNT(*)
FROM Course_Signup csn
GROUP BY
course_id
HAVING COUNT(*) >= 10
Note: you should also GROUP BY the primary key the uniquely identifies the course as there may be two courses with the same name.
If you also don't want to use GROUP BY then:
SELECT name
FROM Courses c
WHERE 10 <= ( SELECT COUNT(*)
FROM Course_Signup csn
WHERE csn.course_id = c.id )
or, to also get the number of sign-ups:
SELECT *
FROM (
SELECT name,
( SELECT COUNT(*)
FROM Course_Signup csn
WHERE csn.course_id = c.id ) AS num_signups
FROM Courses c
)
WHERE num_signups >= 10;
You could do:
SELECT c.name
FROM Courses c
WHERE (
SELECT COUNT(*)
FROM Course_Signup csn
WHERE csn.course_id = c.id
) >= 10
which only uses a subquery and has no group-by, join or set operations.
fiddle
If you wanted the actual count in the result set then you would need to repeat the subquery in the select list.
You might also need to do COUNT(DISTINCT cs.student_id) if there might be duplicates; particularly if the same student can sign up in multiple years - but then you might want to restrict to a single year anyway.
I can select maximum the score of each department but I can't show the name of each person associated with the max score.
I tried to select the name and the maximum grade (with maximum function) but it doesn't work:
select max(stgrade)as highscore,StName,DepName --department
from TBL_DEPARTMANTS d
inner join TBL_LESSONS l on d.DepID=l.LessonID
inner join TBL_GRADES g on g.lessonid=l.LessonID
inner join TBL_STUDENT s on s.STID=g.stid
group by DepName,StName
order by DepName,highscore desc
You may try this...
select * from ( select rank() over (partition by DepName order by stgrade desc) as Slno, stgrade, stname, DepName
from TBL_DEPARTMANTS d
inner join TBL_LESSONS l on d.DepID=l.LessonID
inner join TBL_GRADES g on g.lessonid=l.LessonID
inner join TBL_STUDENT s on s.STID=g.stid ) as dep where dep.slno=1
First create rank() in decreasing order of grade for individual department. then select top record for same.
Note: Use RANK() or DENSE_RANK(), both will work fine for top 1 record, while if you want to select n highest grade then use DENSE_RANK(), at the last for slno pass n'th record you want to select.
Always hard to do theory selects, but while DarkRob's solution is good, it will remove students if for instance two people are best. This is why I love using cross apply:
select
d.Depname
, s.StName
, g.stgrade
from TBL_DEPARTMANTS d
inner join TBL_LESSONS l on
d.DepID=l.LessonID
inner join TBL_GRADES g on
g.lessonid=l.LessonID
inner join TBL_STUDENT s on
s.STID=g.stid
cross apply (
select
sub_d.DepID
, max(sub_g.stgrade) as maxgrade
from TBL_DEPARTMANTS sub_d
inner join TBL_LESSONS sub_l on
sub_d.DepID=sub_l.LessonID
inner join TBL_GRADES sub_g on
sub_g.lessonid=sub_l.LessonID
where sub_d.Dep_ID = d.Dep_ID
group by sub_d.DepID
) as sub
where g.stgrade = sub.maxgrade
This way you will get all the people with max grade per department and not just one.
I have three tables;
1. Students - id, name
2. Subjects - sid,sname
3. Results - id,sid,marks (id and sid are foreign keys referenced by the two tables above)
Now, i perform
SELECT s.sname AS SubjectName, MAX(r.marks) AS MaxMarks
FROM subjects s, results r
WHERE s.sid=r.sid
GROUP BY r.sid, s.sname
ORDER BY r.sid
and i get The Subject Name with the maximum marks scored in them.
Now further, i also want the student name that has scored these max marks.
So i tried adding the column r.id, didn't work. I tried adding the table students in this query. I'm probably goofing up with the grouping after adding the table or something?
I did this
SELECT r.id AS StudentID, s.sname AS SubjectName, MAX(r.marks) AS MaxMarks
FROM subjects s, results r
WHERE s.sid=r.sid
GROUP BY r.sid, s.sname, r.id
ORDER BY r.sid
and i got each StudentID, with repeated subjects and the marks scored.
Whereas what i basically want is the student who has scored the highest in each subject.
you may use ROW_NUMBER() to tag the student who marked the highest on each subject
SELECT st.name,
sj.sname,
r.marks
FROM (SELECT id,
sid,
marks,
ROW_NUMBER() OVER (PARTITION BY sid
ORDER BY marks DESC) maxmarks
FROM results) r
JOIN students st
ON r.id = st.id
JOIN subjects sj
ON r.sid = sj.sid
WHERE r.maxmarks = 1
you can simple add the new join, and i suggest you the use of explicit join sintax
SELECT t.name, s.sname AS SubjectName, MAX(r.marks) AS MaxMarks
FROM subjects s
INNER JOIN results r ON s.sid=r.sid
INNER JOIN Students t ON t.id = r.id
GROUP BY r.sid, s.sname, t.name
ORDER BY r.sid
In the implicit join sintax should be
SELECT s.sname AS SubjectName, MAX(r.marks) AS MaxMarks
FROM subjects s, results r, stundet t
WHERE s.sid=r.sid
and t.id = r.id
GROUP BY r.sid, s.sname, t.name
ORDER BY r.sid
select s.name, s.id
from student s join takes t on t.id = s.id
where s.name like 'D%'
group by s.name, s.id
having (
select count(distinct c.course_id)
from course c
where c.dept_name = 'History' and c.course_id = t.course_id)>4
order by s.name
I am confused about how GROUP BY works. I am trying to find the students who has taken at least 5 courses from history department and name start with D.
Not sure with the nested subqueries...
course(course id, title, dept name, credits)
student(ID, name, dept name, tot_cred)
takes(ID, course_id, sec_id, semester, year, grade)
You have to additionally JOIN with course table:
select s.name, s.id
from student s
inner join takes t on t.id = s.id
inner join course c on c.course_id = t.course_id
where s.name like 'D%' and c.dept_name = 'History'
group by s.name, s.id
having count(distinct c.course_id) >= 5
The WHERE clause returns all students whose names start with a 'D' and have taken at least one course in history department. The HAVING clause filters out any students with 4 or less distinct courses in history department.
i have 2 tables called
Location (id, name)
Person (id, name, location_id)
A person has a location Id which joins these tables . . i would like a SQL query that gives me each location and the count of person table for that id.
i could do something like this and then add up the records in code but i want to find out a way that i only get one row per region with count of people in that region
SELECT l.*, r.id from Location l
inner join Person r
on r.location_id = l.id
order by l.name asc
You want to use aggregates and the GROUP BY clause
SELECT l.id, l.name, count(r.id)
FROM Location l
INNER JOIN Person r on r.location_id = l.id
GROUP BY l.id., l.name
ORDER BY l.name asc
Try:
Select L.Name, Count(*) PersonsCount
From Location L
Join Person P On P.Location_Id = L.Id
Group By L.Name
or if you want to see Locations with zero counts,
Select L.Name, Count(*) PersonsCount
From Location L
Left Join Person P On P.Location_Id = L.Id
Group By L.Name
SELECT lo.name, COUNT(*)
FROM LOCATION lo
JOIN PERSON p ON p.location_id = lo.id
GROUP BY lo.name
ORDER BY lo.name
try this
select count(*), Location.name, Location.id from Location, Person where Person.location_id = Location.id group by Location.id