资料来源 : Free On-Line Dictionary of Computing

outer join
     
         A less commonly used variant of the {inner join}
        {relational database} operation.  An inner join selects rows
        from two {tables} such that the value in one {column} of the
        first table also appears in a certain column of the second
        table.  For an outer join, the result also includes all rows
        from the first operand ("left outer join", "*="), or the
        second operand ("right outer join", "=*"), or both ("full
        outer join", "*=*").  A field in a result row will be null if
        the corresponding input table did not contain a matching row.
     
        For example, if we want to list all employees and their
        employee number, but not all employees have a number, then we
        could say (in {SQL}):
     
        	SELECT employee.name, empnum.number
        	WHERE employee.id *= empnum.id
     
        The "*=" means "left outer join" and means that all rows from
        the "employee" table will appear in the result, even if there
        is no match for their ID in the empnum table.
     
        (2001-03-23)