I am designing a database to store XML elements and attributes.

Create a table as this:

table ELEMENT
ELEM-ID       number(9) primary key,
ELEM-NAME     char(30),
ELEM-CONTENTS char(30),
PARENT-ID     number(9), (references another ELEM-ID)

table ATTRIBUTE
ATTR-ID       number(9) primary key,
ATTR-NAME     char(30),
ATTR-VALUE    char(30),
ELEM-OWNER-ID number(9), (refrences an ELEM-ID)

Elements with null in PARENT-ID are root's elements, so you can make a query selecting null in PARENT-ID to get the list of root's elements.

To get all the children of an element you have to select all elements having parent-id = your elem-id.

Attributes are referenced 1 to many with Elements.