How to do a complicated SQLite Join with Count - sqlite

Question

With SQLite, Given the tables below
_id Name
1 StudentA
2 StudentB
3 StudentC
and
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
or in SQL Form
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;
I would like to show which students have missed which tests and by how many times
The SQL query below gets me close but not exactly what I need
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
Gives me the results table:-
Name Test Count
StudentA A 2
StudentA B 2
StudentB A 2
StudentB B 1
StudentC NULL 0
BUT I would like the table as shown below:-
Name Test Count
StudentA A 2
StudentA B 2
StudentB A 2
StudentB B 1
StudentC A 0
StudentC B 0
Is there any way to do this with SQLite?

Answer 1

Cross join the table Students with the distinct Tests and then left join to the table 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
See the demo.
Or without the 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
See the demo.
Results:
| Name | Test | Count |
| -------- | ---- | ----- |
| StudentA | A | 2 |
| StudentA | B | 2 |
| StudentB | A | 2 |
| StudentB | B | 1 |
| StudentC | A | 0 |
| StudentC | B | 0 |