Mark Levene George Loizou Semantics for Null Extended Nested Relations. 414-459 1993 18 ACM Trans. Database Syst. 3 db/journals/tods/tods18.html#LeveneL93 TODS18/P414.PDF http://doi.acm.org/10.1145/155271.155275 journals/jcss/AbiteboulB86 ... conf/pods/AtzeniB87 journals/iandc/AtzeniM86 conf/adbt/BeeriV79 journals/jcss/Bidoit87 journals/tcs/BunemanJO91 journals/tods/Codd79 journals/sigmod/Codd86 conf/sigmod/Colby89 journals/tods/Delobel78 journals/tods/FaginMU82 journals/jacm/Fagin83 journals/jcss/FischerSTG85 journals/jacm/GrahamMV86 ... journals/jacm/GyssensPG89 ... journals/jacm/Honeyman82 journals/jacm/ImielinskiL84 conf/pods/JaeschkeS82 journals/bit/JajodiaS90 conf/pods/KolaitisP88 conf/oodbs/Korth88 journals/tcs/LeratL86 ... conf/nf2/LeveneL87 journals/cj/LeveneL90 journals/fuin/LeveneL93 ... conf/vldb/Lien79 journals/jacm/Lien82 journals/tods/Lipski79 journals/tods/MaierMS79 books/cs/Maier83 journals/tods/MaierUV84 conf/vldb/Makinouchi77 journals/tods/Mendelzon84 journals/ipl/MillerGKL88 ... journals/tods/OzsoyogluY87 conf/nf2/OzsoyogluY87 journals/tods/RothKS88 journals/acta/RothKS89 journals/tods/Sagiv83 journals/siamcomp/Sagiv88 journals/is/SchekS86 ... journals/jacm/Sciore82 conf/pods/SteinM85 conf/nf2/Takeda87 journals/acr/ThomasF86 books/cs/Ullman88 journals/jcss/GuchtF88 journals/jcss/Zaniolo84