Mysql – Find the second highest value from a subquery or derived table

greatest-n-per-groupMySQLsubquery

I have a subquery which returns the following results.

'c003', '120000.00'
'c002', '90000.00'
'c001', '20000.00'
'c005', '8000.00'
'c004', '5000.00'

I want to find the second highest value from this. How can it be done?

The subquery is :

SELECT SUM(coursefees)
FROM enrollment e INNER JOIN batch b INNER JOIN course c
ON e.batchid = b.batchid AND b.courseid = c.courseid
GROUP BY c.courseid;

Best Answer

Write your join properly and just limit the result set after sorting if it's enough for you to get the second value, no matter if there are two or more courses with the same sum of coursefees. If you need to consider this, it becomes more complicated. Just let me know...

SELECT SUM(coursefees) AS sum_coursefees
FROM enrollment e INNER JOIN batch b ON e.batchid = b.batchid
INNER JOIN course c ON b.courseid = c.courseid
GROUP BY c.courseid
ORDER BY sum_coursefees DESC
LIMIT 1 OFFSET 1;

UPDATE:

That should do it:

SELECT * FROM (
  SELECT 
  c.courseid, 
  @rank := IF(@prev = SUM(coursefees), @rank, @rank + 1) AS rank,
  @prev := SUM(coursefees) AS sum_coursefees
  FROM enrollment e 
  INNER JOIN batch b ON e.batchid = b.batchid
  INNER JOIN course c ON b.courseid = c.courseid
  , (SELECT @rank:=0, @prev:=NULL) var_init
  GROUP BY c.courseid
  ORDER BY sum_coursefees DESC
) sq
WHERE rank = 2;

Note though, that the order in the select clause is important. Do not switch columns.

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