Michael V. Mannino Extensional/Intensional Database Systems. 65-66 1990 ER db/conf/er/er90.html#Manino90 conf/er/90 db/conf/er/Manino90.html er90/ER90-P065.pdf journals/computer/Conklin87 books/aw/Date90 journals/cacm/KaplanKBLD90