MySQL 8.0

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select version();


version()


8.0.36


CREATE TABLE sales_data (
  Sale int,
  Department varchar(10)
);

INSERT INTO sales_data VALUES
(5, 'A'),
(4, 'A'),
(2, 'B'),
(3, 'A');

Records: 4  Duplicates: 0  Warnings: 0


SELECT t.Department,
       COALESCE(SUM(Sale), 0) as Total
FROM (
  SELECT 'A' AS Department
  UNION ALL SELECT 'B'
  UNION ALL SELECT 'C'
) AS t 
LEFT JOIN sales_data s ON s.Department = t.Department
GROUP BY t.Department


Department
Total


A
12

B
2

C
0