Question

I keep my customer phone and fax numbers in the same database table. Which isn’t abnormal.
Not all my customers have phone numbers, while others do not have fax numbers.

I need to view both phone and faxes, even if they’re empty.
In the tbl_customers table I have 2 specific columns dedicated to the record’s default phone and default fax. They simply store the record id from tbl_phonenumbers.

Here’s for phone:

SELECT c.customername, p.phonenumber
FROM `tbl_customers` c
LEFT JOIN `tbl_phonenumbers` p ON c.customerid = p.customerid
WHERE c.defaultphone = 22

Now I need to add the default faxnumber id, which follows the same idea as the default phone.

I’m not quite sure how to go about this. Of course, I could create an entirely new database table called ‘tbl_faxnumbers` and LEFT JOIN to that. But I’d rather keep one single table for phone and fax numbers.

Is this a possibility with the LEFT JOIN?

Answer 1

You can join the same table multiple times, as long as each join has a unique alias so the DB server can figure out WHICH of those extra versions of the table you’re talking about:

SELECT ...
FROM maintable
JOIN othertable AS copy1 ON copy1.somefield = ...
JOIN othertable AS copy2 ON copy2.somefield = ...

For your case, I’d add a “type” identifier field to the phone numbers table, so you can indicate which is a phone number and which is a fax number. Then you’d do your joins as:

SELECT ..., phones.phonenumber, faxes.phonenumber
...
JOIN tbl_phonenumbers AS phones ON (... = phones.id) AND (phones.type = 'phone')
JOIN tbl_phonenumbers AS faxes ON (... = faxes.id) AND (faxes.type = 'fax');