nAllows us to name, and therefore to refer to, the results
of relational-algebra expressions.
nAllows us to refer to a relation by more than one name.
nExample:
r x (E)
returns the
expression E under the name X
nIf a relational-algebra expression E has arity n,
then
returns the result
of expression E under the name X, and
with the
attributes renamed
to A1 , A2 , …., An
.
Banking Example
branch
(branch_name, branch_city, assets)
customer
(customer_name, customer_street, customer_city)
account
(account_number, branch_name, balance)
loan (loan_number, branch_name, amount)
depositor
(customer_name, account_number)
borrower (customer_name, loan_number)
No comments:
Post a Comment