As GN said when this thread was young, a good few ballads in the Child canon have no known tunes, and of those some were probably literary compositions which had never been sung.  Traditional tunes exist for other variants of some (see Bronson's Traditional Tunes of the Child Ballads in particular) and they are still occasionally coming to light, but in many cases it is quite impossible to tell whether or not similar tunes were used for the texts which were originally noted without tunes... bit of a minefield, really.