[大数据]Mysql datetime取日期day，取每月最后一天，取每月天数

mysql> select day('2020-01-01 18:10:10')
    -> ;
+----------------------------+
| day('2020-01-01 18:10:10') |
+----------------------------+
|                          1 |
+----------------------------+
1 row in set (0.00 sec)

mysql> select day('2020-01-21 18:10:10')
    -> ;
+----------------------------+
| day('2020-01-21 18:10:10') |
+----------------------------+
|                         21 |
+----------------------------+
1 row in set (0.00 sec)

mysql> select day('2020-10-21 18:10:10')
    -> ;
+----------------------------+
| day('2020-10-21 18:10:10') |
+----------------------------+
|                         21 |
+----------------------------+
1 row in set (0.00 sec)

mysql> select last_day('2021-01-01 12:12:01');
+---------------------------------+
| last_day('2021-01-01 12:12:01') |
+---------------------------------+
| 2021-01-31                      |
+---------------------------------+
1 row in set (0.00 sec)

mysql> ;
ERROR: 
No query specified

mysql> select last_day('2021-09-01 12:12:01');
+---------------------------------+
| last_day('2021-09-01 12:12:01') |
+---------------------------------+
| 2021-09-30                      |
+---------------------------------+
1 row in set (0.00 sec)

mysql>