Convergence a list of SQL statements of two lines of time difference formed by different conditions

SQL statement that asks for two lines of time difference between different conditions in the same column
Table

CREATE TABLE ' Hp_report ' (
' id ' int (ten) unsigned not NULL auto_increment,
' Code ' varchar (255) is not NULL,
' Content ' Mediumtext not NULL,
' CTime ' datetime not NULL,
) Engine=myisam DEFAULT Charset=utf8 auto_increment=662555;


INSERT into ' Hp_report ' VALUES (2, ' a ', ' on ', ' 2014-07-04 21:17:53 ');
INSERT into ' Hp_report ' VALUES (3, ' A ', ' abc ', ' 2014-07-04 21:18:53 ');
INSERT into ' Hp_report ' VALUES (4, ' A ', ' off ', ' 2014-07-04 21:19:53 ');
INSERT into ' Hp_report ' VALUES (5, ' B ', ' on ', ' 2014-07-04 21:20:53 ');
INSERT into ' Hp_report ' VALUES (6, ' B ', ' abc ', ' 2014-07-04 21:22:53 ');
INSERT into ' Hp_report ' VALUES (7, ' B ', ' off ', ' 2014-07-04 21:29:53 ');
INSERT into ' Hp_report ' VALUES (8, ' a ', ' on ', ' 2014-07-04 21:34:53 ');
INSERT into ' Hp_report ' VALUES (9, ' a ', ' abc ', ' 2014-07-04 21:36:53 ');
INSERT into ' hp_report ' VALUES (' A ', ' off ', ' 2014-07-04 21:45:53 ');
INSERT into ' Hp_report ' VALUES (one, ' B ', ' on ', ' 2014-07-04 22:12:53 ');
INSERT into ' hp_report ' VALUES (' B ', ' abc ', ' 2014-07-04 22:18:53 ');
INSERT into ' hp_report ' VALUES (+, ' B ', ' off ', ' 2014-07-04 22:19:53 ');

I want to find out the difference between a and b,content for the ctime of off, and on, and sum the difference
That

INSERT into ' Hp_report ' VALUES (4,  ' A ', ' off ',  ' 2014-07-04 21:19:53 ');
And
INSERT into ' Hp_report ' VALUES (2,  ' A ', ' on ',  ' 2014-07-04 21:17:53 ');
difference between CTime (2 minutes)

INSERT into ' Hp_report ' VALUES (8,  ' A ', ' on ',  ' 2014-07-04 21:34:53 ');

INSERT into ' Hp_report ' VALUES (  ' A ', ' off ',  ' 2014-07-04 21:45:53 ');
This is 11 minutes.

and add up all the difference in a.
The result I want is
A  13
b   16

Ask the great gods to help. Thank you!!
------Solution--------------------
Can we take the time out and calculate it later?
Strtotime ($row [' CTime ']);

Echo (Strtotime (' 2014-07-04 21:19:53 ')-strtotime (' 2014-07-04 21:17:53 '));
The result is 120 units of seconds
------Solution--------------------

Mysql> select * from Hp_report;
+----+------+---------+---------------------+

------Solution--------------------
Id
------Solution--------------------
Code
------Solution--------------------
Content
------Solution--------------------
CTime
------Solution--------------------

+----+------+---------+---------------------+

------Solution--------------------
2
------Solution--------------------
A
------Solution--------------------
On
------Solution--------------------
2014-07-04 21:17:53
------Solution--------------------


------Solution--------------------
3
------Solution--------------------
A
------Solution--------------------
Abc
------Solution--------------------
2014-07-04 21:18:53
------Solution--------------------


------Solution--------------------
4
------Solution--------------------
A
------Solution--------------------
Off
------Solution--------------------
2014-07-04 21:19:53
------Solution--------------------


------Solution--------------------
5
------Solution--------------------
B
------Solution--------------------
On
------Solution--------------------
2014-07-04 21:20:53
------Solution--------------------


------Solution--------------------
6
------Solution--------------------
B
------Solution--------------------
Abc
------Solution--------------------
2014-07-04 21:22:53
------Solution--------------------
