JPOX should be fixed to run a SQL like

Class A (COL1, COL2, COL3) - TABLE A

Class B extends A (COL4, COL5, COL6) - TABLE B

Class C extends A (COL7, COL8, COL9) - TABLE C

Class D extends C (COL10) - TABLE D

Class E extends D (COL11) - TABLE D

SELECT COL1, COL2, COL3, NULL AS COL4, NULL AS COL5, NULL AS COL6, NULL AS COL7, NULL AS COL8, NULL AS COL9, NULL AS COL10, NULL AS COL11 FROM A WHERE ...

UNION

SELECT COL1, COL2, COL3, COL4, COL5, COL6, NULL AS COL7, NULL AS COL8, NULL AS COL9, NULL AS COL10, NULL AS COL11 FROM A JOIN B WHERE ...

UNION

SELECT COL1, COL2, COL3, NULL AS COL4, NULL AS COL5, NULL AS COL6, COL7, COL8, COL9, NULL AS COL10, NULL AS COL11 FROM A JOIN C WHERE ...

UNION

SELECT COL1, COL2, COL3, NULL AS COL4, NULL AS COL5, NULL AS COL6, COL7, COL8, COL9, COL10, NULL AS COL11 FROM A JOIN C JOIN D WHERE ...

UNION

SELECT COL1, COL2, COL3, NULL AS COL4, NULL AS COL5, NULL AS COL6, COL7, COL8, COL9, COL10, COL11 FROM A JOIN C JOIN D WHERE ...