Selecting distinct values from multiple columns

Question

I have a mail table with four columns: id, userto, userfrom, and message. I want to receive a list of users that have either sent mail to user "example", and I want to join it with a list of users that user "example" sent mail to. However, I don't want any repeating usernames in this list.

How would I do this? I'm sure it has something to do with SELECT DISTINCT and INNER JOIN, but I'm not sure how I would go about doing it.

Politank-Z · Accepted Answer · 2015-04-17 18:39:08Z

I would go with a UNION. Your mileage may vary depending on your RDBMS.

SELECT userto AS listuser FROM mail WHERE userfrom = 'example' UNION SELECT userfrom AS listuser FROM mail WHERE userto = 'example';

Edit: You don't need to use SELECT DISTINCT there, UNION will get rid of repetition from within each subquery as well as between the queries.

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