ZDataQueryKit - QueryJoins: Choose how the datasources are linked

    Description

        This form allows you to choose how the datasources you have
        selected should be joined together. Normally this will be via
        *Key Fields* for instance a field with 'ID' in its name.

        You can normally find information about these joins in the
        documentation that came with your database. Most database
        designers will make the joined fields fairly obvious though.

    Controls

        'Check Box' -- Select which join(s) should are to be removed.

        'Left Column' -- Select the column that should be on the left
        of the relationship.

        'Relation' -- Choose how the columns are related. (See **Relation Operators** below)

        'Right Column' -- Select the column that should be on the right
        of the relationship.

        'Remove' -- Remove the select join(s).

        'Add' -- Add a new join with the chosen settings.

    Note:

        It is a general requirement of this system that at least one
        datasource must appear only on the left, not at all. If a
        datasource can not be found that obeys this rule, an error will
        be raised.

    Relation Operations

        The the supported relational operations are::

            =       Equal
            !=      Not equal
            <       Less than
            >       Greater than
            <=      Less than or equal
            >=      Greater than or equal

        In addition to this list there are two special operators for
        ZVisualQuery and ZVisualODBCClient::

            =*      Equal or no matching record in right column
            *=      Equal or no matching record in left column

        These last two operators are quite deep into SQL voodoo, and
        should be avoided unless you know what an *OUTER JOIN* is.
