1.terminologies:
attributes(column in table) table(relation) domain(permitted value)
tuple(row)
database shema && database instance --------------->>>> type definition && variable
2.key
superkey (can uniquely mark a tuple in a relation) ---->>>> candidate key (its subset cannot be superkey)
---->>>>primary key(designer chosen)
3.relational-Algebra operation
a. select b.project
c.union: here are two conditions to hold for validaty
i. the same number of attributes(the same arity); ii.corresponding attribute is the same
d. - (con) e. × f.rename
g.∩ h.natural join(theta) i.÷(condition) j.assignment <-
~Extended~ Generalized projection aggregate function(max, min, count, count-distinct, avg, sum...)
outer join(left, right, full) null values
~Database modification~
deletion(set difference) updating(select, projection) insertion(union)
just a daily review or extraction : P