Displaying total logged time of a tree in org-mode

In org-mode, I often log time for tasks with C-c-C-x-C-i (org-clock-in) and C-c C-x C-o (org-clock-out). At the end of the day, it is nice to be able to sum up times for all tasks.

	
Top task 1
	
* Sub task 1

   :LOGBOOK:
   CLOCK: [2018-08-29 on. 09:05]--[2018-08-29 on. 11:37] =>  2:32
   :END:

	
* Sub task 2

   :LOGBOOK:
   CLOCK: [2018-08-30 to. 13:14]--[2018-08-30 to. 14:38] =>  1:24
   CLOCK: [2018-08-30 to. 09:46]--[2018-08-30 to. 11:36] =>  1:50
   :END:

	
Top task 2
	
* Sub task 3

   :LOGBOOK:
   CLOCK: [2018-08-30 to. 13:14]--[2018-08-30 to. 14:38] =>  1:24
   CLOCK: [2018-08-30 to. 09:46]--[2018-08-30 to. 11:36] =>  1:50
   :END:

	
* Sub task 4

   :LOGBOOK:
   CLOCK: [2018-08-30 to. 21:02]--[2018-08-30 to. 21:02] =>  0:00
   CLOCK: [2018-08-30 to. 14:38]--[2018-08-30 to. 15:08] =>  0:30
   :END:

Luckily, org-mode contains a short cut for this: C-c C-x C-d. The command displays the sum of all times:

	
Top task 1···············································        5:46
	
* Sub task 1··············································        2:32

   :LOGBOOK:
   CLOCK: [2018-08-29 on. 09:05]--[2018-08-29 on. 11:37] =>  2:32
   :END:

	
* Sub task 2··············································        3:14

   :LOGBOOK:
   CLOCK: [2018-08-30 to. 13:14]--[2018-08-30 to. 14:38] =>  1:24
   CLOCK: [2018-08-30 to. 09:46]--[2018-08-30 to. 11:36] =>  1:50
   :END:

	
Top task 2···············································        3:44
	
* Sub task 3··············································        3:14

   :LOGBOOK:
   CLOCK: [2018-08-30 to. 13:14]--[2018-08-30 to. 14:38] =>  1:24
   CLOCK: [2018-08-30 to. 09:46]--[2018-08-30 to. 11:36] =>  1:50
   :END:

	
* Sub task 4··············································        0:30

   :LOGBOOK:
   CLOCK: [2018-08-30 to. 21:02]--[2018-08-30 to. 21:02] =>  0:00
   CLOCK: [2018-08-30 to. 14:38]--[2018-08-30 to. 15:08] =>  0:30
   :END:

Another useful command is C-c C-c which updates a LOGBOOK row after you manually edited it.