选择月份、年份和数量，并显示零条目的月份[重复]

Question

我有以下 mysql 查询：

SELECT 
LEFT(MONTHNAME(date), 3) AS month,
YEAR(date) AS year,
COUNT(id) AS quantity
FROM
budgets
GROUP BY MONTH(date)

返回给我这个：

|month|year|quantity|
---------------------
|  Jan|2018|      10|
|  Mar|2018|       2|
|  May|2018|       5|

即使COUNT(id) 不存在，我也需要显示月份 像这样：

|month|year|quantity|
---------------------
|  Jan|2018|      10|
|  Feb|2018|       0|
|  Mar|2018|       2|
|  Apr|2018|       0|
|  May|2018|       5|

有人知道怎么做吗？

更新

根据 D-Shih 答案代码，查询返回：

|month|year|quantity|
---------------------
|  Jan|2018|      10|
|  Feb|null|       0|
|  Mar|2018|       2|
|  Apr|null|       0|
|  May|2018|       5|

如果我添加一个不同年份的寄存器，它将只显示一个名称为 2017 的行，例如：

|month|year|quantity|
---------------------
|  Jan|2018|      10|
|  Feb|null|       0|
|  Mar|2018|       2|
|  Apr|null|       0|
|  May|2018|       5|
|  May|2017|       2|

它必须显示

|month|year|quantity|
---------------------
|  Jan|2018|      10|
|  Feb|2018|       0|
|  Mar|2018|       2|
|  Apr|2018|       0|
|  May|2018|       5|
|  Jan|2017|       0|
|  Feb|2017|       0|
|  Mar|2017|       0|
|  Apr|2017|       0|
|  May|2017|       2|

Answer 1

你可以尝试做一个日历表和OUTER JOIN基于日历表

SELECT t1.month,
       y.year AS year,
       COUNT(t2.id) 
FROM (
    SELECT 'Jan' month
    UNION ALL
    SELECT 'Feb' month
    UNION ALL
    SELECT 'Mar' month
    UNION ALL
    SELECT 'Apr' month
    UNION ALL
    SELECT 'May' month
) t1 
CROSS JOIN (SELECT DISTINCT YEAR(date) FROM budgets) y  
LEFT JOIN budgets t2 on t1.month = LEFT(MONTHNAME(t2.date), 3) 
   AND y.year = YEAR(t2.date)
GROUP BY t1.month, y.year

---

这是给您的示例。

架构 (MySQL v8.0)

CREATE TABLE budgets (
   `month` varchar(5),
   `year` int,
   quantity int
);


insert into budgets values ('Jan',2018,10);
insert into budgets values ('Mar',2018,2);
insert into budgets values ('May',2018,5);
insert into budgets values ('May',2017,2);

---

查询 #1

| month | year | quantity |
| ----- | ---- | -------- |
| Jan   | 2018 | 10       |
| Mar   | 2018 | 2        |
| May   | 2018 | 5        |
| May   | 2017 | 2        |
| Jan   | 2017 | 0        |
| Feb   | 2018 | 0        |
| Feb   | 2017 | 0        |
| Mar   | 2017 | 0        |
| Apr   | 2018 | 0        |
| Apr   | 2017 | 0        |

---

View on DB Fiddle

Answer 2

如果您想扩大规模，我认为这应该可行。不过不太确定它的效率：

SELECT a.month AS month, a.year AS year, COALESCE(b.quantity,0) AS quantity
FROM
(
   SELECT DISTINCT MONTH(date) AS month, YEAR(date) AS year
   FROM
   budgets
) a
LEFT JOIN
(
   SELECT MONTH(date) AS month, YEAR(date) AS year,
   COUNT(id) AS quantity
   FROM
   budgets
   GROUP BY MONTH(date), YEAR(date)
) b
ON a.month = b.month AND a.year = b.year