Question

I want to learn the comment displaying algorithm behind Reddit. How is a comment related with its child and so on? How they are stored in the database?

Lets say

comment1
-comment2
--comment3
-comment4
--comment5
--comment6
---comment7
----comment8
comment9

How to display comment5 which is after comment4 which is after comment1? What is the idea behind this sequencing? And how to relate them in the database?

Answer 1

AS @Rafe said, the actual storage is pretty easy, it would be something like:

|  id  |   name   | parent |
|  1   | comment1 |    0   |
|  2   | comment2 |    1   |
|  3   | comment3 |    2   |
|  4   | comment4 |    1   |
|  5   | comment5 |    4   |
|  6   | comment6 |    4   |
|  7   | comment7 |    6   |
|  8   | comment8 |    7   |
|  9   | comment9 |    0   |

Of course actually getting information from this is (arguably) the hard part. You can of course get the children of a comment with something like: SELECT * FROM table WHERE parent='4' will give you all the children of comment4. But counting children, listing all the children in hierarchical order would be a bit harder. Other answers may provide more information on that.