MySQL 如何在MySQL中计算天数？

首先创建一个包含一个日期时间列和另一个存储天数的列的表：

mysql> create table DemoTable
(
   ShippingDate datetime,
   CountOfDate int
);
Query OK, 0 rows affected (0.52 sec)

使用插入命令插入一些记录，如下所示：

mysql> insert into DemoTable values('2018-01-31',6);
Query OK, 1 row affected (0.11 sec)
mysql> insert into DemoTable values('2016-12-01',15);
Query OK, 1 row affected (0.21 sec)
mysql> insert into DemoTable values('2016-12-01',10);
Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable values('2016-12-01',5);
Query OK, 1 row affected (0.17 sec)
mysql> insert into DemoTable values('2019-04-09',3);
Query OK, 1 row affected (1.05 sec)
mysql> insert into DemoTable values('2018-01-31',4);
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable values('2019-04-09',4);
Query OK, 1 row affected (0.14 sec)

使用select命令从表中显示记录的查询如下：

mysql> select *from DemoTable;

这将产生以下带日期和计数的输出：

+---------------------+-------------+
| ShippingDate        | CountOfDate |
+---------------------+-------------+
| 2018-01-31 00:00:00 |           6 |
| 2016-12-01 00:00:00 |          15 |
| 2016-12-01 00:00:00 |          10 |
| 2016-12-01 00:00:00 |           5 |
| 2019-04-09 00:00:00 |           3 |
| 2018-01-31 00:00:00 |           4 |
| 2019-04-09 00:00:00 |           4 |
+---------------------+-------------+
7 rows in set (0.00 sec)

使用group by date(ShippingDate)查询每天的总和如下：

mysql> select date(ShippingDate),sum(CountOfDate) from DemoTable group by date(ShippingDate);

这将产生以下输出：

+--------------------+------------------+
| date(ShippingDate) | sum(CountOfDate) |
+--------------------+------------------+
| 2018-01-31         |               10 |
| 2016-12-01         |               30 |
| 2019-04-09         |                7 |
+--------------------+------------------+
3 rows in set (0.00 sec)

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