mysql - 如何选择表行值作为值总和的列

Question

我有下表:

CREATE TABLE products
(
date DATE,
productname VARCHAR(80),
quantity INT(5)
);

INSERT INTO `products`(`date`, `productname`, `quantity`) VALUES ('2016-12-16','toy',5);
INSERT INTO `products`(`date`, `productname`, `quantity`) VALUES ('2016-12-18','santa',8);
INSERT INTO `products`(`date`, `productname`, `quantity`) VALUES ('2016-12-23','tree',15);
INSERT INTO `products`(`date`, `productname`, `quantity`) VALUES ('2016-11-16','toy',5);
INSERT INTO `products`(`date`, `productname`, `quantity`) VALUES ('2016-11-19','santa',2);
INSERT INTO `products`(`date`, `productname`, `quantity`) VALUES ('2016-11-24','tree',5);
INSERT INTO `products`(`date`, `productname`, `quantity`) VALUES ('2016-10-01','toy',10);
INSERT INTO `products`(`date`, `productname`, `quantity`) VALUES ('2016-10-04','santa',20);
INSERT INTO `products`(`date`, `productname`, `quantity`) VALUES ('2016-10-09','tree',30);
INSERT INTO `products`(`date`, `productname`, `quantity`) VALUES ('2016-10-01','toy',40);
INSERT INTO `products`(`date`, `productname`, `quantity`) VALUES ('2016-10-04','santa',30);
INSERT INTO `products`(`date`, `productname`, `quantity`) VALUES ('2016-10-09','tree',20)

我想查看数量总和，每个月的日期为每行一个，产品名称为列，因此我创建了如下查询:

SELECT DATE_FORMAT(`date`, '%Y-%m') As Date, 
IF(`productname` = 'santa', SUM(`quantity`), 'none') As santa,
IF(`productname` = 'toy', SUM(`quantity`), 'none') As toy,
IF(`productname` = 'tree', SUM(`quantity`), 'none') As tree  

FROM `products`

GROUP BY DATE_FORMAT(`date`, '%Y-%m'),`productname`

这给了我这样的东西:

+---------+-------+------+------+
| Date    | santa | toy  | tree |
+---------+-------+------+------+
| 2016-10 | 50    | none | none |
+---------+-------+------+------+
| 2016-10 | none  | 50   | none |
+---------+-------+------+------+
| 2016-10 | none  | none | 50   |
+---------+-------+------+------+
| 2016-11 | 2     | none | none |
+---------+-------+------+------+
| 2016-11 | none  | 5    | none |
+---------+-------+------+------+
| 2016-11 | none  | none | 5    |
+---------+-------+------+------+
| 2016-12 | 8     | none | none |
+---------+-------+------+------+
| 2016-12 | none  | 5    | none |
+---------+-------+------+------+
| 2016-12 | none  | none | 15   |
+---------+-------+------+------+

这几乎不错，但我希望它是这样的，所以特定月份只有一行:

+---------+-------+------+------+
| Date    | santa | toy  | tree |
+---------+-------+------+------+
| 2016-10 | 50    | 50   | 50   |
+---------+-------+------+------+
| 2016-11 | 2     | 5    | 5    |
+---------+-------+------+------+
| 2016-12 | 8     | 5    | 15   |
+---------+-------+------+------+

是否可以通过查询来实现？

Answer 1

应该这样做

SELECT DATE_FORMAT(`date`, '%Y-%m') As Date,
  IFNULL(Sum(Case when `productname` = 'santa' then `quantity` end),0) As santa, 
  IFNULL(Sum(Case when `productname` = 'toy' then `quantity` end),0) As toy, 
  IFNULL(Sum(Case when `productname` = 'tree' then `quantity` end),0) As tree 
FROM `products`
GROUP BY DATE_FORMAT(`date`, '%Y-%m');