representing graph using relational database

Q1 : Is there a way to represent variable elements in a [database] table?

I assume you mean something like this?

 from | to_1 | to_2 | to_3 | to_4 | to_5 | etc... 1    | 2    | 3    | 4    | NULL | NULL | etc...

This is not a good idea. It violates first normal form.

Q2 : Is there any general way to represent the graph structure using database?

For a directed graph you can use a table edges with two columns:

nodeid_from nodeid_to1           21           31           4

If there is any extra information about each node (such as a node name) this can be stored in another table nodes.

If your graph is undirected you have two choices:

• store both directions (i.e. store 1->2 and 2->1)
• use a constraint that nodeid_from must be less than nodeid_to (i.e. store 1->2 but 2->1 is implied).

The former requires twice the storage space but can make querying easier and faster.

In addition to the two tables route mentioned by Mark take a look at the following link:

http://articles.sitepoint.com/article/hierarchical-data-database/2

This article basically preorders the elements in the tree assigning left and right values. You are then able to select portions or all of the tree using a single select statement.

Node | lft | rght-----------------  A  |  0  |  7  B  |  1  |  2  C  |  3  |  4  D  |  5  |  6

EDIT: If you are going to be updating the tree heavily this is not an optimum solution as the whole tree must be re-numbered

I have stored multiple "TO" nodes in a relational representation of a graph structure. I was able to do this because my graph was directed. This meant that if I wanted to know what nodes "A" was connected to, I only needed to select a single record from my table of connections. I stored the TO nodes in an easy-to-parse string and it worked great, with a class that could manage the conversion from string to collection and back.