how to sort the data based on average number of two columns in sql server?

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How to sort the data based on average no. of occurances..
studentid, professorid, lectureid (these are my column names....)
I need to sort the data based on average no.of students per professor...
For ex: A professor might have 65 students, with 4 different lectureids, the average is 16, like this which professors average is highest, that professor students must be listed first)

I am able to get the professorid and average count of students in another table, but I am not able to sort it in the same table!!..And using-- count(*) over(partition by professorid) as [studentcount] this statement I am able to sort the data based on highiest count..but not able to do averaging..

studentid   professorid  lectureid
1            101          401
2            101          402  
3            101          401
4            101          402
5            102          403
6            102          404
7            103          405
8            104          406

Posted 26-Dec-13 1:47am

2012programmer

Updated 26-Dec-13 2:22am

v2

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King Fisher 26-Dec-13 8:10am

can u show the table structure here with example datas

2 solutions

Solution 1

Based on your sample data, i think you need to use ORDER BY with aggregate function, like: COUNT().

SQL

DECLARE @tmp TABLE(studentid INT IDENTITY(1,1), professorid INT, lectureid INT)
 
INSERT INTO @tmp (professorid, lectureid)
VALUES(101, 401), (101, 402),
(101, 401), (101, 402),
(102, 403), (102, 404),
(103, 405), (104, 406)
 

SELECT professorid, CountOfStudents / CountOfLectures AS AvgCount
FROM (
	SELECT professorid, COUNT(studentid) AS CountOfStudents, COUNT(DISTINCT lectureid) AS CountOfLectures
	FROM @tmp
	GROUP BY professorid
) AS T
ORDER BY CountOfStudents / CountOfLectures  DESC

Posted 26-Dec-13 7:30am

Maciej Los

Updated 26-Dec-13 9:47am

v2

Comments

2012programmer 26-Dec-13 15:07pm

I need the all the studentids sorted by maximumu avergestudents....
1. Professor=101 has 4 lectureids=401,411,412,415, and each lectureid has various no. of students(25,26,20,19)..
The averagecount=Totalno.ofstudents(25+26+20+19)/no.of distinct lectureids(4)=23
2. Professor=102 has 3 lectureids=416,417,419 and each lectureid has various no. of students(20,19,21)..
The averagecount=Totalno.ofstudents(20+19+21)/no.of distinct lectureids(3)=20
....
Like this there will be 60 professors, with different averagecount...My table should be sorted in a way that, If professor=119 has averagecount=24(for ex: highest)..then all the students of this professor should appear first..

Maciej Los 26-Dec-13 15:47pm

Have a look at updated answer ;)

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Christian Graus · Accepted Answer · 2013-12-26T09:36:00

Use a CTE to calculate your averages, then join to that CTE to create an order by

SQL

DECLARE @tmp TABLE(studentid INT IDENTITY(1,1), professorid INT, lectureid INT)

INSERT INTO @tmp (professorid, lectureid)
VALUES(101, 401), (101, 402),(101, 402),(101, 402),(101, 402),(101, 402),
(101, 401), (101, 402),
(102, 403), (102, 404),(102, 404),(102, 404),(102, 404),
(103, 405), (104, 406);

select * from @tmp;

with avgStudents as
(
 select professorId, count(studentid)/count( distinct lectureid) as average from @tmp group by professorId
)

select t.studentid, av.professorid, t.lectureid, av.average from @tmp t
inner join avgStudents av on av.professorId = t.professorId
order by av.average desc