MySQL版本，可參考

思路:

1. 產生需要月份的 table(可以用臨時表)，這邊直接抓表中的(不嚴謹，因為表沒有就會少日期)
2. 產生需要產品的 table(可以用臨時表)，這邊直接抓表中的(不嚴謹，因為表沒有就會少產品)
3. 將上述兩張 table join 就可以得到需要的報表項目(日期 + 產品)
4. 對數量 table left join，再判斷如果是 null 就給 0

SELECT 
    name_table.`name`,
    month_table.`date`,
    IFNULL(demo.`count`, 0) AS `count`
FROM
    (
		SELECT 
			`date`
		FROM
			demo
		GROUP BY `date`
    ) month_table
    INNER JOIN
    (
		SELECT 
			`name`
		FROM
			demo
		GROUP BY `name`
    ) name_table
    LEFT JOIN
		demo ON 
			month_table.`date` = demo.`date` AND 
            name_table.`name` = demo.`name`
ORDER BY 
	name_table.`name`,
    month_table.`date`

CREATE TABLE `demo` (
  `id` int NOT NULL AUTO_INCREMENT,
  `name` varchar(45) COLLATE utf8_unicode_ci NOT NULL,
  `date` varchar(45) COLLATE utf8_unicode_ci NOT NULL,
  `count` int NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

INSERT INTO `demo` VALUES (1,'紅茶','2020-01',800),(2,'紅茶','2020-02',750),(3,'紅茶','2020-03',900),(4,'綠茶','2020-01',900),(5,'綠茶','2020-02',850),(6,'綠茶','2020-03',900),(7,'青茶','2020-01',700),(8,'青茶','2020-03',900);

有幫助到你請給個最佳解答

看錯題目了
自刪

不是一個欄位 NULL 要顯示成 0
而是「少了一筆資料」要補進去
青茶 2021-02 0

我不會

CREATE TABLE TableX(
A text(20), -- 品名
B text(20), -- 月份
C int);     -- 銷售量

INSERT INTO TableX (A,B,C) values 
('紅茶','2021-01',800),
('紅茶','2021-02',750),
('紅茶','2021-03',900),
('綠茶','2021-01',900),
('綠茶','2021-02',850),
('綠茶','2021-03',900),
('青茶','2021-01',700),
('青茶','2021-03',900);

CREATE TABLE TableY(
A text(20)); -- 品名

INSERT INTO TableY (A) values 
('紅茶'),
('綠茶'),
('青茶'),
-- 額外增加
('烏龍茶');

-- MySQL 8
SELECT TempK.A AS '品名',TempM.MonthID AS '月份',IFNULL(TableX.C, 0) AS '銷售量'
FROM (
WITH RECURSIVE cte_count (n) 
AS (
      SELECT 1
      UNION ALL
      SELECT n + 1 
      FROM cte_count 
      WHERE n < 12
    )
SELECT n,
CONCAT('2021-',LPAD(CONCAT(n, ''),2,0)) AS 'MonthID'
FROM cte_count
-- 計算到 3 月
WHERE n<=3
) AS TempM
LEFT JOIN (
SELECT A 
FROM TableY
) AS TempK ON 1=1
LEFT JOIN TableX ON TableX.B=TempM.MonthID AND TableX.A=TempK.A
ORDER BY TempK.A,TempM.MonthID

-- 非 MySQL 8
SELECT TempK.A AS '品名',TempM.MonthID AS '月份',IFNULL(TableX.C, 0) AS '銷售量'
FROM (
-- 計算到 3 月
SELECT '2021-01' AS 'MonthID'
UNION
SELECT '2021-02' AS 'MonthID'
UNION
SELECT '2021-03' AS 'MonthID'
) AS TempM
LEFT JOIN (
SELECT A 
FROM TableY
) AS TempK ON 1=1
LEFT JOIN TableX ON TableX.B=TempM.MonthID AND TableX.A=TempK.A
ORDER BY TempK.A,TempM.MonthID

Demo