SELECT content FROM html WHERE url="http://en.wikipedia.org/wiki/List_of_United_States_National_Parks_by_state" AND xpath="//a/text()"
動作しませんが、
SELECT * FROM html WHERE url="http://en.wikipedia.org/wiki/List_of_United_States_National_Parks_by_state" AND xpath="//a/text()"
します。
SELECT content FROM html WHERE url="http://en.wikipedia.org/wiki/List_of_United_States_National_Parks_by_state" AND xpath="//a"
YQL にバグがあるようですが、何か不足していますか?