A
      /
     B
    /
   C
  /
 D

##</code><code>##

 id | ancestor | depth
 A  |  NULL    |   0
 B  |  A       |   1
 C  |  A       |   2
 C  |  B       |   1
 D  |  A       |   3
 D  |  B       |   2
 D  |  C       |   1