Как сделать сложное соединение SQLite с помощью Count

Question

С SQLite, учитывая таблицы ниже

_id Name
1   StudentA
2   StudentB
3   StudentC

и

id  StudentId   Test    Score
1   1           A       5
2   1           B       5
3   1           A       6
4   1           B       6
5   2           A       3
6   2           B       3
7   2           A       4

или в SQL форме

BEGIN TRANSACTION;
DROP TABLE IF EXISTS "Results";
CREATE TABLE IF NOT EXISTS "Results" (
    "id"    INTEGER PRIMARY KEY AUTOINCREMENT,
    "StudentId" INTEGER,
    "Test"  TEXT,
    "Score" INTEGER,
    FOREIGN KEY("StudentId") REFERENCES "Students"("_id")
);
DROP TABLE IF EXISTS "Students";
CREATE TABLE IF NOT EXISTS "Students" (
    "_id"   INTEGER PRIMARY KEY AUTOINCREMENT,
    "Name"  TEXT NOT NULL
);
INSERT INTO "Results" ("id","StudentId","Test","Score") VALUES (1,1,'A',5);
INSERT INTO "Results" ("id","StudentId","Test","Score") VALUES (2,1,'B',5);
INSERT INTO "Results" ("id","StudentId","Test","Score") VALUES (3,1,'A',6);
INSERT INTO "Results" ("id","StudentId","Test","Score") VALUES (4,1,'B',6);
INSERT INTO "Results" ("id","StudentId","Test","Score") VALUES (5,2,'A',3);
INSERT INTO "Results" ("id","StudentId","Test","Score") VALUES (6,2,'B',3);
INSERT INTO "Results" ("id","StudentId","Test","Score") VALUES (7,2,'A',4);
INSERT INTO "Students" ("_id","Name") VALUES (1,'StudentA');
INSERT INTO "Students" ("_id","Name") VALUES (2,'StudentB');
INSERT INTO "Students" ("_id","Name") VALUES (3,'StudentC');
COMMIT;

Я хотел бы показать, какие студенты пропустили какие тесты и сколько раз

Приведенный ниже запрос SQL приближает меня, но не совсем то, что мне нужно

SELECT s.Name, r.Test, COUNT(r.Test) AS Count
FROM Students s
LEFT OUTER JOIN Results r ON  s._id = r.StudentId
GROUP BY s._id, r.Test

Дает мне таблицу результатов: -

Name        Test    Count
StudentA    A       2
StudentA    B       2
StudentB    A       2
StudentB    B       1
StudentC    NULL    0

НО мне нужна таблица, показанная ниже: -

Name        Test    Count
StudentA    A       2
StudentA    B       2
StudentB    A       2
StudentB    B       1
StudentC    A       0
StudentC    B       0

Есть ли способ сделать это с SQLite?

Answer 1

Крест соедините таблицу Students с различными Test с, а затем оставьте соединение с таблицей Results:

WITH cte AS (
  SELECT *
  FROM Students 
  CROSS JOIN (SELECT DISTINCT Test FROM Results) 
)  
SELECT c.Name, c.Test, COUNT(r.Test) AS Count
FROM cte c  
LEFT OUTER JOIN Results r ON  c._id = r.StudentId AND c.Test = r.Test
GROUP BY c.Name, c.Test

См. Демонстрационную версию . Или без CTE:

SELECT s.Name, t.Test, COUNT(r.Test) AS Count
FROM Students s 
CROSS JOIN (SELECT DISTINCT Test FROM Results) t
LEFT OUTER JOIN Results r ON  s._id = r.StudentId AND t.Test = r.Test
GROUP BY s.Name, t.Test

См. Демоверсию .

Результаты:

| Name     | Test | Count |
| -------- | ---- | ----- |
| StudentA | A    | 2     |
| StudentA | B    | 2     |
| StudentB | A    | 2     |
| StudentB | B    | 1     |
| StudentC | A    | 0     |
| StudentC | B    | 0     |