Hi novice,
>Whats the purpose of joins ?? is it the same as selecting from mulitple tables ?
>
>
In a Relational Database liek Postgresql, you try the store data which
as few redundancy as possible.
The result is that you store the data in many tables - eg. persons and
adresses will be separate
(this process is called Normalisation).
Now, to put the corresponding data together again, you need a join
between the tables that belong together.
In order to work, you need at least (numberOfTables -1) Join-Conditions,
for exaple
SELECT
person.name,
address.line1
FROM
person,
address
WHERE
' Here comes the Join Condition
address.person_id = person.person_id
If you do not specify the Join Condition, you will get all
possible combinations of persons and addresses,
the so called cartesian Product of the 2 tables.
Cheers, Dani
