SQLite 3.27

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CREATE TABLE business
    ("stars" INTEGER,
    "city" varchar(10))
;

INSERT INTO business
    ("stars","city")
VALUES
    (2.5,'Avon')
;


INSERT INTO business
    ("stars","city")
VALUES
    (4.0,'Avon')
;

INSERT INTO business
    ("stars","city")
VALUES
    (5.0,'Avon')
;

INSERT INTO business
    ("stars","city")
VALUES
    (3.5,'Avon')
;

INSERT INTO business
    ("stars","city")
VALUES
    (1.5,'Avon')
;

INSERT INTO business
    ("stars","city")
VALUES
    (3.5,'Avon')
;


INSERT INTO business
    ("stars","city")
VALUES
    (4.5,'Avon')
;
INSERT INTO business
    ("stars","city")
VALUES
    (3.5,'Avon')
;

INSERT INTO business
    ("stars","city")
VALUES
    (2.5,'Avon')
;
INSERT INTO business
    ("stars","city")
VALUES
    (4.0,'Avon')
;
SELECT 
  t2."stars",  IFNULL(ratio_count,0) ratio_count
FROM 
  (select a as stars from
(select 0.5 as a union select 1 union select 1.5 
union select 2 union select 2.5  union select 3 
union select 3.5 union select 4 union select 4.5 union select 5 )) t2 
LEFT JOIN (SELECT stars, COUNT(*) ratio_count FROM business
where city = 'Avon'   GROUP BY  "stars") t1 
ON t2.stars = t1.stars


stars
ratio_count


0.5
0

1
0

1.5
1

2
0

2.5
2

3
0

3.5
3

4
2

4.5
1

5
1