Question

I am trying to query 2 tables using the following sql statement in an attempt to return all records from each table that contains a specific id.

SELECT Phone.Phone, Email.Email FROM Contacts.Phone, Contacts.Email 
WHERE Phone.ContactId = :contactId AND Email.ContactId = :contactId

Contacts.Phone table contains 2 phone numbers for the given id and Contacts.Email contains 1 email for the given id. Using the above sql query, I get the following rows returned. This is just an example, of course, of a situation in where my result set from each table does match in number of rows.

Row 1: 555-555-5555 - [e-mail]
Row 2: 666-666-6666 - [e-mail]

The email is repeated in order to fill in the second row when I am trying to get:

Row 1: 555-555-5555 - [e-mail]
Row 2: 666-666-6666 - NULL

I think I need to use a UNION to somehow join the tables, but I can’t figure out exactly how to write the sql statement. Another option would be to perform 2 separate SQL queries, which would be easier but I figure performance wise it would be better to collect all data I need in one query.

I am using MySQL.

Answer 1

First of all, I think this:

Another option would be to perform 2 separate SQL queries, which would be easier but I figure performance wise it would be better to collect all data I need in one query.

is a bit misguided. The performance difference between one complex query and two simple queries will be quite small. That said, you suggested using a UNION; if you do want to use a single query, you can do this:

SELECT 'EMAIL', Email.Email
  FROM Contacts.Email
 WHERE Email.ContactId = :contactId
UNION ALL
SELECT 'PHONE', Phone.Phone
  FROM Contacts.Phone
 WHERE Phone.ContactId = :contactId
 ORDER BY 1 -- put e-mail addresses before phone-numbers
;

The first column will return the “type” of result, and the second column will have the datum.