multiple record in a single row - sql

Question

I have post graduation degree records of students in my database. student may have only one post graduation degree, some students may have more than one post graduation degree.
rollno | pgdegree | score
--------------------------
0001 | 41 | 56
0002 | 42 | 78
0002 | 49 | 75
0003 | 48 | 77
Here roll no. 0002 is more than one time and roll no. 0001,0003 are only one time.
i want my desired output as :
rollno | pgdegree1 | score1 | pgdegree2 | score2
------------------------------------------------
0001 | 41 | 56 | |
0002 | 42 | 78 | 49 | 75
0003 | 48 | 77 | |
Note : in my database any student can have one or two post gradation only. Not more than two PG degree.

Answer 1

Here is another solution using ROW_NUMBER() and conditional aggregation to save some unnecessary SELECTs :
SELECT s.rollno,
MAX(CASE WHEN s.rnk = 1 THEN s.pgdegree END) AS pgdegree1,
MAX(CASE WHEN s.rnk = 1 THEN s.score END) AS score1,
MAX(CASE WHEN s.rnk = 2 THEN s.pgdegree END) AS pgdegree2,
MAX(CASE WHEN s.rnk = 2 THEN s.score END) AS score2
FROM
(
SELECT t.*,
ROW_NUMBER() OVER (PARTITION BY t.rollno ORDER BY t.pgdegree, t.score) AS rnk
FROM YourTable t
) s
GROUP BY s.rollno

Answer 2

Do a self LEFT JOIN to add second pgdegree if available for a rollno. Do NOT EXISTS to only return rows with lowest pgdegree as t1.pgdegree.
select t1.rollno, t1.pgdegree, t1.score, t2.pgdegree, t2.score
from tablename t1
left join tablename t2
on t1.rollno = t2.rollno and t1.pgdegree < t2.pgdegree
where not exists (select * from tablename t3
where t1.rollno = t3.rollno
and t1.pgdegree > t3.pgdegree)

Answer 3

You can use a clever pivot query:
SELECT t.rollno,
SUM(CASE WHEN pgdegree = (SELECT MIN(pgdegree) FROM yourTable WHERE rollno = t.rollno)
THEN pgdegree ELSE 0 END) AS pgdegree1,
SUM(CASE WHEN pgdegree = (SELECT MIN(pgdegree) FROM yourTable WHERE rollno = t.rollno)
THEN score ELSE 0 END) AS score1,
SUM(CASE WHEN pgdegree = (SELECT MAX(pgdegree) FROM yourTable WHERE rollno = t.rollno)
THEN pgdegree ELSE 0 END) AS pgdegree2,
SUM(CASE WHEN pgdegree = (SELECT MAX(pgdegree) FROM yourTable WHERE rollno = t.rollno)
THEN score ELSE 0 END) AS score2
FROM yourTable t
GROUP BY t.rollno
Explanation:
The first two CASE statements have subqueries which will return pgdegree if that value happens to be the minimum value for that given rollno. This pgdegree and score will appear as the first two columns. Similarly, the last two CASE statements use the maximum value to generate the second two columns.

Answer 4

select
rollno,
(array_agg(pgdegree))[1] as pgdegree1,
(array_agg(score))[1] as score1,
(array_agg(pgdegree))[2] as pgdegree2,
(array_agg(score))[2] as score2
from
your_table
group by
rollno;