Perhaps there is a mis-understanding between metadata queries and data
queries.
First is the metadata query, asking which columns are appropriate. This would be done in Xquery or ADQL (as previously discussed), and it returns a small result. It could be in terms of UCD matching, or even Google type matching in the column descriptions.
Once we know which columns we want, and what they mean, we can issu a data query. A data query must specify exactly what is being done, and may return a lot of data. It would be in SQL or something very like it (eg ADQL).
Roy