RA
Declarative language/Procudual language
RA:operands,operator
select/projection can be composited together;projection will select only those distinct tuples, which is equivalant to select distinct column name.
cross production:combines 2 database together.;will match automatically on 1 attribute.
set difference:
union:both relation mush have the same schema(attr形式相同,will filter duplicate rows)
renaming:pa1,a2(name):自己的relation和自己join的时候需要进行一种区分.