XSL: Accessing child or parent node

For example, accessing the child node which the column_node_01 equal to 'a' :
<xsl value="parent_node[child_column_node_01='a']/child_column_node_02"/>

From child node, accessing the parent node, as easy as :
<xsl value="../../parent_column_node_01"/>

It's easy playing with XML/XSL.... :)