If you use SQL2, just correct your query like that
SELECT * FROM [my:type] WHERE PATH([my:type]) LIKE '/content/subtree/%'
For SQL, you should use :
SELECT * FROM my:Type WHERE jcr:path like '/content/subtree/%'
this query will search all node in all level under "/content/subtree/"
but if you only need to search on one level under "subtree", you should to add another option to your query in order to increase the performance :
SELECT * FROM my:Type WHERE jcr:path like '/content/subtree/%' and not jcr:path like 'content/subtree/%/%'
You can found more detail here : http://docs.jboss.org/exojcr/1.12.13-GA/developer/en-US/html/ch-jcr-query-usecases.html#JCR.ChildNodeConstraint