GROUP BY EXTRACT(YEAR_MONTH_DAY_HOUR FROM date_time)

GROUP BY CONCAT(EXTRACT(YEAR_MONTH_DAY FROM date_time), EXTRACT(HOUR FROM date_time))

GROUP BY LEFT(FROM_UNIXTIME(date_time), 13)

GROUP BY LEFT(date_time, 13)

create table t(id int, id_tag int, value int, date_time int);

mysql> create table test(field timestamp);
Query OK, 0 rows affected (0.19 sec)

mysql> insert into test values (12345678);
ERROR 1292 (22007): Incorrect datetime value: '12345678' for column 'field' at row 1
mysql> insert into test values ('2013-01-01 01:02:03');
Query OK, 1 row affected (0.02 sec)

mysql> select * from test;
+---------------------+
| field               |
+---------------------+
| 2013-01-01 01:02:03 |
+---------------------+
1 row in set (0.00 sec)

mysql> drop table test;
Query OK, 0 rows affected (0.05 sec)

mysql> create table test(field int);
Query OK, 0 rows affected (0.22 sec)

mysql> insert into test values (12345678);
Query OK, 1 row affected (0.03 sec)

mysql> insert into test values ('2013-01-01 01:02:03');
ERROR 1265 (01000): Data truncated for column 'field' at row 1
mysql> select * from test;
+----------+
| field    |
+----------+
| 12345678 |
+----------+
1 row in set (0.00 sec)

mysql> drop table test;
Query OK, 0 rows affected (0.05 sec)

SELECT *, CAST(date_time AS DATETIME) FROM t;

SELECT SUM(value) Summ, CONCAT(LEFT(FROM_UNIXTIME(date_time), 13),':00:00') Hour
FROM t
WHERE id_tag=1
GROUP BY LEFT(FROM_UNIXTIME(date_time), 13);