mysql通过订单创建时间按年统计各市1到12月份的订单量，优化版本

你可以这样写 SQL 语句：

SELECT city,
       SUM(CASE WHEN month = 1 THEN num_orders ELSE 0 END) AS 'Jan',
       SUM(CASE WHEN month = 2 THEN num_orders ELSE 0 END) AS 'Feb',
       SUM(CASE WHEN month = 3 THEN num_orders ELSE 0 END) AS 'Mar',
       SUM(CASE WHEN month = 4 THEN num_orders ELSE 0 END) AS 'Apr',
       SUM(CASE WHEN month = 5 THEN num_orders ELSE 0 END) AS 'May',
       SUM(CASE WHEN month = 6 THEN num_orders ELSE 0 END) AS 'Jun',
       SUM(CASE WHEN month = 7 THEN num_orders ELSE 0 END) AS 'Jul',
       SUM(CASE WHEN month = 8 THEN num_orders ELSE 0 END) AS 'Aug',
       SUM(CASE WHEN month = 9 THEN num_orders ELSE 0 END) AS 'Sep',
       SUM(CASE WHEN month = 10 THEN num_orders ELSE 0 END) AS 'Oct',
       SUM(CASE WHEN month = 11 THEN num_orders ELSE 0 END) AS 'Nov',
       SUM(CASE WHEN month = 12 THEN num_orders ELSE 0 END) AS 'Dec'
FROM orders
WHERE year = ? -- 这里的 ? 是占位符，查询时需要替换成真实的年份
GROUP BY city;

假设表 `orders` 中有如下几列：

- `id`：订单 ID，数字型。
- `city`：订单所在市，字符型。
- `year`：订单创建的年份，数字型。
- `month`：订单创建的月份，数字型。
- `num_orders`：订单数量，数字型。

这条 SQL 语句会按市分组，统计出每个市在每个月的订单数量。

希望这个答案对你有所帮助！