PostgreSQL-query met dynamische kolommen en tellingen van join

Eenvoudige zoekopdracht voor "Pogingen",

select student_id,sum(case when assessment_id=1 then 1 else 0 end) as "Assessment 1",
sum(case when assessment_id=2 then 1 else 0 end) as "Assessment 2",
sum(case when assessment_id=3 then 1 else 0 end) as "Assessment 3",
sum(case when assessment_id=4 then 1 else 0 end) as "Assessment 4",
sum(case when assessment_id=5 then 1 else 0 end) as "Assessment 5",
sum(case when assessment_id=6 then 1 else 0 end) as "Assessment 6"
from assessments_students
group by student_id
order by student_id

In de functie kruistabel() moet u ook de kolomnaam expliciet definiëren, zoals "Assessment 1","Assessment 2" enzovoort.

Of schrijf een aangepaste functie voor het maken van dynamische query's en voer deze uit met de instructie EXECUTE.

DROP FUNCTION get_Attempts() ;

CREATE OR REPLACE FUNCTION get_Attempts() RETURNS text AS
$BODY$
DECLARE
        r1 record;
        str_query text := '';
 BEGIN
    str_query :='select student_id,';
    FOR r1 IN SELECT "_id" , "name" FROM Assessments
    LOOP
      str_query:= str_query || 
                  'sum(case when assessment_id=' || r1."_id" || ' then 1 else 0 end) as  "' || r1.name ||'",' ;
    END LOOP;
    str_query:=trim( trailing  ',' from str_query); -- remove last semicolon
    str_query:= str_query || ' from assessments_students group by student_id order by student_id';
    return str_query;
END
$BODY$
LANGUAGE 'plpgsql' ;

SELECT * FROM get_Attempts();

Tweede zoekopdracht voor "geslaagd"

select student_id,
max(case when assessment_id=1 and passed='t' then 't' else 'f' end) as  "Assessment 1",
max(case when assessment_id=2 and passed='t' then 't' else 'f' end) as  "Assessment 2",
max(case when assessment_id=3 and passed='t' then 't' else 'f' end) as  "Assessment 3",
max(case when assessment_id=4 and passed='t' then 't' else 'f' end) as  "Assessment 4",
max(case when assessment_id=5 and passed='t' then 't' else 'f' end) as  "Assessment 5",
max(case when assessment_id=6 and passed='t' then 't' else 'f' end) as  "Assessment 6" 
from assessments_students 
group by student_id 
order by student_id

en zijn functie ziet er leuk uit,

DROP FUNCTION get_passed() ;

CREATE OR REPLACE FUNCTION get_passed() RETURNS text AS
$BODY$
DECLARE
        r1 record;
        str_query text := '';
 BEGIN
    str_query :='select student_id,';
    FOR r1 IN SELECT "_id" , "name" FROM Assessments
    LOOP
      str_query:= str_query || 
                  'max(case when assessment_id=' || r1."_id" || ' and passed=''t'' then ''t'' else ''f'' end) as  "' || r1.name ||'",' ;
    END LOOP;
    str_query:=trim( trailing  ',' from str_query); -- remove last semicolon
    str_query:= str_query || ' from assessments_students group by student_id order by student_id';

    return str_query;
END
$BODY$
LANGUAGE 'plpgsql' ;

SELECT * FROM get_passed();