Selecting the Root Nodes v12

The START WITH clause is used to determine the row(s) selected by table_expression that are to be used as the root nodes. All rows selected by table_expression where start_expression evaluates to true become a root node of a tree. Thus, the number of potential trees in the result set is equal to the number of root nodes. As a consequence, if the START WITH clause is omitted, then every row returned by table_expression is a root of its own tree.