Virtual Observatory (VO) is a general term for a collection of federated resources that can be used to conduct astronomical research, education
and outreach. 

The International Virtual Observatory Alliance (IVOA) is a global collaboration of separately funded projects to develop standards and
infrastructure that enable VO applications.

General query form of ADQL is:

    SELECT
        [ ALL | DISTINCT ]
        [ TOP unsigned_decimal ]
        {
             * |
             { value_expression [ [AS] column_name ] }, ...
         }
        FROM {
                {
                table_name [ [AS] identifier ] |
                ( SELECT ....) [ [AS] identifier ] |
                table_name [NATURAL]
                    [ INNER | { LEFT | RIGHT | FULL [OUTER] } ]
                    JOIN table_name
                    [ON search_condition | USING ( column_name,...) ]
                },
            ...
            }

        [ WHERE search_condition ]
        [ GROUP BY group_by_term, ... ]
        [ HAVING search_condition ]
        [ ORDER BY
            { order_by_expression } [ ASC | DESC],
            ...
            ]
        [ OFFSET unsigned_decimal ]