SQL & PL/SQL

Dear Experts,

I have a table

Table Name :child_parent

Columns:

AS per the requirement I need out put like

COL1 COL2

A        B

A       C

A       D

I HAVE achieved this by using the following

select x1 ,X2 from child_parent

union all

select X1,X3 from child_parent

union all

select X1,X4 from child_parent

Can we have any other approach with we can achieve in a better query.I appreciate your help in this regard.

Thanks,

Ravi