Constant FIND_UNREFERENCED_CONSENSUS_EXTDOCS 

const FIND_UNREFERENCED_CONSENSUS_EXTDOCS: &str = "
    SELECT filename FROM ExtDocs WHERE
         (type LIKE 'con_%' OR type = 'sha3-256')
    AND NOT EXISTS
         (SELECT digest FROM Consensuses WHERE Consensuses.digest = ExtDocs.digest);";

Query: Find all consensus extdocs that are not referenced in the consensus table.

Note: use of sha3-256 is a synonym for con_% is a workaround.