Blank node in RDF

Question

I just started learning about RDF and my question is: What is the purpose of the blank node?

Example:

ex:John   foaf:knows       _:p1
_:p1      foaf:birthDate   04-21

This means that John knows someone who is born on 04-21.

But I can't understand the purpose.

Answer 1

For your simple example the reason for the blank node would be that you do not know anything else about this person born on 04-21, except that John knowns him or her. You have insufficient knowledge to give him/her a proper URI and simply add the two known statements.

Answer 2

Imagine you know a person (let us call him Derek) who has 3 children. You've seen one of the children and he is a boy. You can say:

ex:derek  ex:hasChild  _:b1 .
_:b1  a  ex:Boy .

This means Derek has a boy. You can tell this to your friend Khadim. Now your friend meets Derek and one of his child. The child is a boy. So Khadim can say:

ex:derek  ex:hasChild  _:b2 .
_:b2  a  ex:Boy .

This means Derek has a boy. It confirms and is equivalent to your statement. However, if you had given an identifier to the boy you've seen, it would be:

ex:derek  ex:hasChild   ex:theBoyIHaveSeen .
ex:theBoyIHaveSeen  a  ex:Boy .

Then you're friend Khadim would say:

ex:derek   ex:hasChild   ex:theBoyYourFriendHasSeen .
ex:theBoyYourFriendHasSeen  a  ex:Boy .

This does not confirm what you are saying as you don't know if the boy you have seen is the same. Yet you would be able to agree on the fact that Derek has a boy. So, with a URI, you are really expressing something different.

Answer 3

A blank node is a node that does not have a URI as its identifier. Simple question come why don't we give/use a URI to that node also, the simple reason is that node-detail (URI) is not having much link/importance to our document/rdf. If particular node details is directly or indirectly link to our content then we should provided/use URI to it.