Truy vấn PostgreSQL với các cột động và số lượng từ phép tham gia

Truy vấn đơn giản cho "Nỗ lực",

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

Trong hàm crosstab (), cần xác định tên cột một cách rõ ràng như "Đánh giá 1", "Đánh giá 2", v.v.

Hoặc viết hàm tùy chỉnh để tạo truy vấn động và thực thi bằng câu lệnh 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();

Truy vấn thứ hai cho "đã qua"

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

và chức năng của nó giống như giao diện,

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();