select * from R where a=1
-- has 1 specified attribute (a)-- has 3 unspecified attributes (b,c,d)
select * from R where b=5 and d=2
-- has 2 specified attributes (b,d)-- has 2 unspecified attributes (a,c)
select * from R
where a=1 and b=5 and c=3 and d=2
-- has 4 specified attributes (a,b,c,d)-- has 0 unspecified attributes