The Natural-Join Operation
It is often
desirable to simplify certain queries that require a Cartesian product.
Usually, a query that involves a Cartesian product includes a selection
operation on the result of the Cartesian Product. Consider the query “Find the
names of all customers who have a loan at the bank, along with the loan number
and the loan amount.”
The natural
join is a binary operation that allows us to combine certain selections and a
Cartesian product into one operation. It is denoted by the join symbol ∞. The
natural-join operation forms a Cartesian product of its two arguments, performs
a selection forcing equality on those attributes that appear in both the
relation schemas and finally removes duplicate attributes.
We express this
query by using the natural join as follows:
- Find the names of all branches with customers who have an account in the bank and who live in Dhaka.
The natural
join operation on the three relations can be executed in any order.
- Find all customers who have both a loan and an account at the bank.
We can write an expression for this query by using
set intersection as follows:
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