MySQL historical data lookup

Question

I've been looking around and trying to get this to work but I can't seem to get it. I have 2 tables:

TABLE: products

| id | name      | some more values |
|----|-----------|------------------|
| 1  | Product 1 | Value 1          |
| 2  | Product 2 | Value 2          |
| 3  | Product 3 | Value 3          |


TABLE: value

| pid | value     | stamp            |
|-----|-----------|------------------|
| 1   | 7         | 2015-07-11       |
| 2   | 4         | 2015-07-11       |
| 3   | 8         | 2015-07-11       |
| 1   | 9         | 2015-07-21       |
| 2   | 4         | 2015-07-21       |
| 3   | 6         | 2015-07-21       |

First table simply has a list of products, second table has a value for each product (by pid), and the timestamp the value. note: timestamps are not every day, nor are they evenly spaced.

What I would like, is a resulting table like this:

| id | name      | some more values | value now | value last month |
|----|-----------|------------------|-----------|------------------|
| 1  | Product 1 | Value 1          | 9         | 7                |
| 2  | Product 2 | Value 2          | 4         | 4                |
| 3  | Product 3 | Value 3          | 6         | 8                |

where 'value now' is the value of the newest timestamp, and the 'value last month' is the value of the timestamp closest to the newest timetamp - 30 days. Keep in mind that -30 days might not have a specific timestamp, the query will need to find the closest timestamp. (looking only up or down doesn't matter, it's an approximation.)

I have made some huge queries but I'm pretty sure there must be an easier way... Any help would be appreciated.

Answer 1

Assuming you get last month and year by PHP or by mysql function, here is a not checked query I hope it will work on first time:

SELECT *, v_now, v_lastmonth FROM products p 
LEFT JOIN (SELECT `value` AS v_now FROM value ORDER BY stamp DESC) AS v_now ON p.id=v_now.pid
LEFT JOIN (SELECT `value` AS v_lastmonth FROM value 
WHERE month(stamp)='$month' AND year(stamp)='$year'
ORDER BY stamp DESC) AS v_now ON p.id=v_now.pid

You can use group by to get one row for each product result.