CREATE TABLE A (  A1 INTEGER, A2 INTEGER  ); CREATE TABLE B (  B1 INTEGER, B2 INTEGER  ); INSERT INTO A ( A1,A2 ) VALUES ( 1,3 );INSERT INTO A ( A1,A2 ) VALUES ( 2,4 );INSERT INTO B ( B1,B2 ) VALUES ( 1,2 );INSERT INTO B ( B1,B2 ) VALUES ( 3,NULL );