mysql：GROUP_CONCAT

首先新建一张订单分批销售表，并插入数据，如下：

一 使用
mysql> select * from order_sale_info;
+----+---------------------+-----+----------+
| id | sale_date           | num | order_id |
+----+---------------------+-----+----------+
|  1 | 2020-05-12 23:28:13 |  12 | 100      |
|  2 | 2020-05-14 23:28:36 |  15 | 100      |
|  3 | 2020-05-15 23:28:51 |   1 | 100      |
|  4 | 2020-05-16 23:29:04 |  10 | 101      |
|  5 | 2020-05-17 00:29:21 |   8 | 101      |
+----+---------------------+-----+----------+
5 rows in set

需求是根据订单号查出分批销售的时间、数量，并将他们拼接为字符串输出，先看语句和结果：

mysql> SELECT order_sale_info.order_id, GROUP_CONCAT(order_sale_info.sale_date,':',order_sale_info.num) saleInfo FROM 
`order_sale_info` GROUP BY order_sale_info.order_id;
+----------+---------------------------------------------------------------------+
| order_id | saleInfo                                                            |
+----------+---------------------------------------------------------------------+
| 100      | 2020-05-12 23:28:13:12,2020-05-14 23:28:36:15,2020-05-15 23:28:51:1 |
| 101      | 2020-05-16 23:29:04:10,2020-05-17 00:29:21:8                        |
+----------+---------------------------------------------------------------------+
2 rows in set

此处就使用了GROUP_CONCAT函数，此函数可以将将符合条件（GROUP BY order_sale_info.order_id）的行数据组合起来（2020-05-12 23:28:13:12,2020-05-14 23:28:36:15,2020-05-15 23:28:51:1 ）。

理解：CONCAT是mysql连接/拼接函数，GROUP是分组的意思，那GROUP_CONCAT就是按条件分组并拼接的意思了。

进一步，如果想查在某个时间段内的数据，可如下：

SELECT order_sale_info.order_id, GROUP_CONCAT(order_sale_info.sale_date,':',order_sale_info.num) saleInfo FROM `order_sale_info` 

WHERE order_sale_info.sale_date BETWEEN '2020-05-13 23:28:13' AND '2020-05-17 13:28:13' GROUP BY order_sale_info.order_id;
+----------+----------------------------------------------+
| order_id | saleInfo                                     |
+----------+----------------------------------------------+
| 100      | 2020-05-14 23:28:36:15,2020-05-15 23:28:51:1 |
| 101      | 2020-05-16 23:29:04:10,2020-05-17 00:29:21:8 |
+----------+----------------------------------------------+
2 rows in set

 二 注意事项

如果GROUP_CONCAT拼接的字段中存在null值，那拼接出来的结果也将是null。例如：

+----+---------------------+------+----------+
| id | sale_date           | num  | order_id |
+----+---------------------+------+----------+
|  1 | 2020-05-12 23:23:12 |   12 |      100 |
|  2 | 2020-05-14 23:23:31 |   15 |      100 |
|  3 | 2020-05-15 23:23:50 |    1 |      100 |
|  4 | 2020-05-16 23:24:14 |   10 |      101 |
|  5 | 2020-05-17 23:24:33 | NULL |      101 |
+----+---------------------+------+----------+

执行语句：

SELECT order_sale_info.order_id, GROUP_CONCAT(order_sale_info.sale_date,':',order_sale_info.num) saleInfo FROM 
`order_sale_info` GROUP BY order_sale_info.order_id;

结果为：

+----------+---------------------------------------------------------------------+
| order_id | saleInfo                                                            |
+----------+---------------------------------------------------------------------+
|      100 | 2020-05-12 23:23:12:12,2020-05-14 23:23:31:15,2020-05-15 23:23:50:1 |
|      101 | 2020-05-16 23:24:14:10                                              |
+----------+---------------------------------------------------------------------+

可以看到有null的这一行（第5行）数据的拼接结果为null，仅有第4行的拼接数据。

那假如想让有null的字段显示为空字符串如何处理呢，使用IFNULL来处理：

SELECT order_sale_info.order_id, GROUP_CONCAT(order_sale_info.sale_date,':',IFNULL(order_sale_info.num,' ')) saleInfo FROM 
`order_sale_info` GROUP BY order_sale_info.order_id;

结果为：

+----------+---------------------------------------------------------------------+
| order_id | saleInfo                                                            |
+----------+---------------------------------------------------------------------+
|      100 | 2020-05-12 23:23:12:12,2020-05-14 23:23:31:15,2020-05-15 23:23:50:1 |
|      101 | 2020-05-16 23:24:14:10,2020-05-17 23:24:33:                         |
+----------+---------------------------------------------------------------------+