How to compare the first date and the last date from a MySQL column with date records?

To compare the first date with the last date, use TIME_TO_SEC() along with MAX() and MIN(). Let us first create a table −

mysql> create table DemoTable
(
   UserName varchar(100),
   UserPostDatetime datetime
);
Query OK, 0 rows affected (0.65 sec)

Insert some records in the table using insert command −

mysql> insert into DemoTable values('Adam','2019-08-24 11:10:00');
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable values('Adam','2019-08-24 11:20:00');
Query OK, 1 row affected (0.17 sec)
mysql> insert into DemoTable values('Adam','2019-08-24 11:50:00');
Query OK, 1 row affected (0.13 sec)

Display all records from the table using select statement −

mysql> select *from DemoTable;

This will produce the following output −

+----------+---------------------+
| UserName | UserPostDatetime    |
+----------+---------------------+
|     Adam | 2019-08-24 11:10:00 |
|     Adam | 2019-08-24 11:20:00 |
|     Adam | 2019-08-24 11:50:00 |
+----------+---------------------+
3 rows in set (0.00 sec)

Following is the query to compare the first date and the last date −

mysql> select UserName,sec_to_time(max(time_to_sec(UserPostDatetime))-min(time_to_sec(UserPostDatetime)))t
from DemoTable
group by UserName;

This will produce the following output −

+----------+----------+
| UserName | t        |
+----------+----------+
|     Adam | 00:40:00 |
+----------+----------+
1 row in set (0.03 sec)