sql grouping grades - sql

Question

I have a table for subjects as follows:
id Subject Grade Ext
100 Math 6 +
100 Science 4 -
100 Hist 3
100 Geo 2 +
100 CompSi 1
I am expecting output per student in a class(id = 100) as follows:
Grade Ext StudentGrade
6 + 1
6 0
6 - 0
5 + 0
5 0
5 - 0
4 + 0
4 0
4 - 1
3 + 0
3 1
3 - 0
2 + 1
2 0
2 - 0
1 + 0
1 1
1 - 0
I would want this done on oracle/sql rather than UI. Any inputs please.

Answer 1

You should generate rows first, before join them with your table like below. I use the with clause here to generate the 18 rows in your sample.
with rws (grade, ext) as (
select ceil(level/3), decode(mod(level, 3), 0, '+', 1, '-', null)
from dual
connect by level <= 3 * 6
)
select r.grade, r.ext, nvl2(t.Ext, 1, 0) studentGrade
from rws r
left join your_table t
on t.Grade = r.Grade and decode(t.Ext, r.Ext, 1, 0) = 1
order by 1 desc, decode(r.ext, null, 2, '-', 3, '+', 1)

Answer 2

You could do something like this. In the WITH clause I generate two small "helper" tables (really, inline views) for grades from 1 to 6 and for "extensions" of +, null and -. In the "extensions" view I also create an "ordering" column to use in ordering the final output (if you are wondering why I included that).
Also in the WITH clause I included sample data - you will have to remove that and instead use your actual table name in the main query.
The idea is to cross-join "grades" and "extensions", and left-outer-join the result to your input data. Count the grades from the input data, grouped by grade and extension, and after filtering the desired id. The decode thing in the join condition is needed because for extension we want to treat null as equal to null - something that decode does nicely.
with
sample_inputs (id, subject, grade, ext) as (
select 100, 'Math' , 6, '+' from dual union all
select 100, 'Science', 4, '-' from dual union all
select 100, 'Hist' , 3, null from dual union all
select 100, 'Geo' , 2, '+' from dual union all
select 100, 'CompSi' , 1, null from dual
)
, g (grade) as (select level from dual connect by level <= 6)
, e (ord, ext) as (
select 1, '+' from dual union all
select 2, null from dual union all
select 3, '-' from dual
)
select g.grade, e.ext, count(t.grade) as studentgrade
from g cross join e left outer join sample_inputs t
on t.grade = g.grade and decode(t.ext, e.ext, 0) = 0
and t.id = 100 -- change this as needed!
group by g.grade, e.ext, e.ord
order by g.grade desc, e.ord
;
OUTPUT:
GRADE EXT STUDENTGRADE
----- --- ------------
6 + 1
6 0
6 - 0
5 + 0
5 0
5 - 0
4 + 0
4 0
4 - 1
3 + 0
3 1
3 - 0
2 + 1
2 0
2 - 0
1 + 0
1 1
1 - 0

Answer 3

It looks like you want sparse data to be filled in as part of joining students and subjects.
Since Oracle 10g the correct way to do this has been with a "partition outer join".
The documentation has examples.
https://docs.oracle.com/en/database/oracle/oracle-database/21/sqlrf/SELECT.html#GUID-CFA006CA-6FF1-4972-821E-6996142A51C6