 /// A "qualified" source, where all tables are identified with a table alias.
 private enum SQLQualifiedSource {
     case table(tableName: String, alias: TableAlias)
-    case commonTableExpression(TableAlias)
     indirect case subquery(SQLQueryGenerator)
     
     /// Nil for subquery sources.

    var alias: TableAlias? {
        switch self {
        case let .table(_, alias):
            return alias
        case let .commonTableExpression(alias):
            return alias
        case .subquery:
            return nil

            } else {
                return "\(tableName.quotedDatabaseIdentifier)"
            }
        case let .subquery(generator):
            let sql = try generator.requestSQL(context)
            return "(\(sql))"
