Question

I have a hypothetical table “users” with the columns

• user_id (auto incremented)
• name
• foo
• bar
• last_updated

This table is updated multiple times per day. How can I query to get the last update, per user, per day, going back X days?

Example Data

1 John a b "2013-01-31 02:01:12"
2 Rich c d "2013-01-31 22:41:12"
3 John e f "2013-01-31 22:01:15"
4 Rich g h "2013-02-01 16:01:12"
5 John i j "2013-02-01 22:21:12"
6 Rich k m "2013-02-01 22:21:12"

Desired Return Set:

2 Rich c d 2013-01-31
3 John e f 2013-01-31
5 John i j 2013-02-01
6 Rich k m 2013-02-01

I am able to get the last updated per user overall with the following query, it’s applying it to each day that I am struggling with.

SELECT u1.*
FROM users u1 
    LEFT JOIN users u2
    ON (u1.name = u2.name AND u1.user_id < u2.user_id)
WHERE u2.user_id IS NULL

Answer 1

First of all the table name users is very confusing, since these aren’t the users but the logins.

Beyond that you’re on the right way. You just need to add a comparison on the date in the join.

SELECT u1.*
FROM users u1
    LEFT JOIN users u2
    ON (u1.name = u2.name AND date_format(u1.last_updated, '%Y-%m-%d') = date_format(u2.last_updated, '%Y-%m-%d') AND u1.user_id < u2.user_id)
WHERE u2.user_id IS NULL

See it work in this SQL fiddle.