How to apply TOP statement to only 1 column while selecting multiple columns from a table? - sql

Question

I am trying to select multiple columns from a table, but I want to select top certain number of records based on one column. I tried this :
select roll_no ,marks as Percentage
from database
where marks in (select top (3) *
from database
where subject = ''
order by marks desc) order by percentage desc
and I am getting the error:
Only one expression can be specified in the select list when the
sub-query is not introduced with EXISTS or more than specified number
of records.
I also tried :
select roll_no ,marks as Percentage
from database
where marks in (select top (3) marks
from database
where subject = ''
order by marks desc) order by percentage desc
which returns the right result for some subjects but for others..it is displaying top marks from other subjects as well.
eg :
+---------+-------+
| roll_no | marks |
+---------+-------+
|10003 | 87 |
|10006 | 72 |
|10003 | 72 |
|10002 | 67 |
|10004 | 67 |
+---------+-------+
How to frame the query correctly?
sample data :
+---------+-------+---------+
| roll_no | marks |subject |
+---------+-------+---------+
|10001 | 45 | Maths |
|10001 | 72 | Science |
|10001 | 64 | English |
|10002 | 52 | Maths |
|10002 | 35 | Science |
|10002 | 75 | English |
|10003 | 52 | Maths |
|10003 | 35 | Science |
|10003 | 75 | English |
|10004 | 52 | Maths |
|10004 | 35 | Science |
|10004 | 75 | English |
+---------+-------+---------+

Answer 1

If I'm right and you are looking for the best 3 marks for each subject, then you can get it with the following:
DECLARE #SelectedSubject VARCHAR(50) = 'Maths'
;WITH FilteredSubjectMarks AS
(
SELECT
D.Subject,
D.Roll_no,
D.Marks,
MarksRanking = DENSE_RANK() OVER (ORDER BY D.Marks DESC)
FROM
[Database] AS D
WHERE
D.Subject = #SelectedSubject
)
SELECT
F.*
FROM
FilteredSubjectMarks AS F
WHERE
F.MarksRanking <= 3

Answer 2

You can use window functions to rank your marks column (specifically dense_rank, which allows duplicate rankings whilst retaining sequential numbering) and then return all rows with a rank of 3 or less:
declare #t table(roll_no int identity(1,1),marks int);
insert into #t(marks) values(2),(4),(5),(8),(6),(1),(3),(2),(1),(8);
with t as
(
select roll_no
,marks
,dense_rank() over (order by marks desc) as r
from #t
)
select *
from t
where r <= 3;
Output:
+---------+-------+---+
| roll_no | marks | r |
+---------+-------+---+
| 4 | 8 | 1 |
| 10 | 6 | 1 |
| 5 | 6 | 2 |
| 3 | 5 | 3 |
+---------+-------+---+