1.0 Background
The Joinable interface provides the methods for getting and
setting a match column, which is the basis for forming the SQL JOIN
formed by adding RowSet objects to a JoinRowSet
object.
Any standard RowSet implementation may implement
the Joinable interface in order to be
added to a JoinRowSet object. Implementing this interface gives
a RowSet object the ability to use Joinable methods,
which set, retrieve, and get information about match columns. An
application may add a
RowSet object that has not implemented the Joinable
interface to a JoinRowSet object, but to do so it must use one
of the JoinRowSet.addRowSet methods that takes both a
RowSet object and a match column or an array of RowSet
objects and an array of match columns.
To get access to the methods in the Joinable interface, a
RowSet object implements at least one of the
five standard RowSet interfaces and also implements the
Joinable interface. In addition, most RowSet
objects extend the BaseRowSet class. For example:
class MyRowSetImpl extends BaseRowSet implements CachedRowSet, Joinable {
:
:
}
2.0 Usage Guidelines
The methods in the Joinable interface allow a RowSet object
to set a match column, retrieve a match column, or unset a match column, which is
the column upon which an SQL JOIN can be based.
An instance of a class that implements these methods can be added to a
JoinRowSet object to allow an SQL JOIN relationship to
be established.
CachedRowSet crs = new MyRowSetImpl();
crs.populate((ResultSet)rs);
(Joinable)crs.setMatchColumnIndex(1);
JoinRowSet jrs = new JoinRowSetImpl();
jrs.addRowSet(crs);
In the previous example, crs is a CachedRowSet object that
has emplemented the Joinable interface. In the following example,
crs2 has not, so it must supply the match column as an argument to the
addRowSet method. This example assumes that column 1 is the match
column.
CachedRowSet crs2 = new MyRowSetImpl();
crs2.populate((ResultSet)rs);
JoinRowSet jrs2 = new JoinRowSetImpl();
jrs2.addRowSet(crs2, 1);
The JoinRowSet interface makes it possible to get data from one or
more RowSet objects consolidated into one table without having to incur
the expense of creating a connection to a database. It is therefore ideally suited
for use by disconnected RowSet objects. Nevertheless, any
RowSet object may implement this interface
regardless of whether it is connected or disconnected. Note that a
JdbcRowSet object, being always connected to its data source, can
become part of an SQL JOIN directly without having to become part
of a JoinRowSet object.
3.0 Managing Multiple Match Columns
The index array passed into the setMatchColumn methods indicates
how many match columns are being set (the length of the array) in addition to
which columns will be used for the match. For example:
int[] i = {1, 2, 4, 7}; // indicates four match columns, with column
// indexes 1, 2, 4, 7 participating in the JOIN.
Joinable.setMatchColumn(i);
Subsequent match columns may be added as follows to a different Joinable
object (a RowSet object that has implemented the Joinable
interface).
int[] w = {3, 2, 5, 3};
Joinable2.setMatchColumn(w);
When an application adds two or more RowSet objects to a
JoinRowSet object, the order of the indexes in the array is
particularly important. Each index of
the array maps directly to the corresponding index of the previously added
RowSet object. If overlap or underlap occurs, the match column
data is maintained in the event an additional Joinable RowSet is
added and needs to relate to the match column data. Therefore, applications
can set multiple match columns in any order, but
this order has a direct effect on the outcome of the SQL JOIN.
This assertion applies in exactly the same manner when column names are used
rather than column indexes to indicate match columns. |
