public void unsetMatchColumn(String columnName) throws SQLException {
        // check if we are unsetting the same column
        columnName = columnName.trim();

        if(!((strMatchColumns.get(0)).equals(columnName))) {
            throw new SQLException(resBundle.handleGetObject("cachedrowsetimpl.unsetmatch").toString());
        } else if(iMatchColumns.get(0) > 0) {
            throw new SQLException(resBundle.handleGetObject("cachedrowsetimpl.unsetmatch2").toString());
        } else {
            strMatchColumns.set(0, null);   // that is, we are unsetting it.
        }
    }
Unsets the designated parameter to the given <code>String</code>
object.  This was set using <code>setMatchColumn</code>
as the column which will form the basis of the join.
<P>
The parameter value unset by this method should be same
as was set.

@param columnName the index into this rowset
       object's internal representation of parameter values
@throws SQLException if an error occurs or the
 parameter index is out of bounds or if the columnName is
 not the same as set using <code>setMatchColumn(String)</code>
src/main/java/org/replicadb/rowset/sun/rowset/CachedRowSetImpl.java