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?
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.