MySQL QUERY - loop through table and perform calculations with conditions (for PHP)

Question

I'm working on one project and now I'm in blind spot, trying to resolve this..

I have MySQL table with name x_world with some columns. Here is an example:

 internalid     name     population           date
          1     John            100     2014-05-01
          2    Tommy            250     2014-05-01
          3     Paul            200     2014-05-01
          4    Holly            700     2014-05-01
          5     John            100     2014-05-02
          6    Tommy            240     2014-05-02
          7     Paul            201     2014-05-02
          8    Holly            680     2014-05-02
          9     John            100     2014-05-03
         10    Tommy            230     2014-05-03
         11     Paul            202     2014-05-03
         12    Holly            799     2014-05-03

What i need, is get this result:

 name     population     (+-1d)     (+-3d)
 John            100         0          0
Tommy            230       -10        -20
 Paul            202        +1         +2

Here is explanation about what i mean and need:

• Max allowed population change (grow) is 2 (including 2). If population grows over 2, dont display this row in result.
• If population is only descending (no matter how much), it's okay, display this row.
• Combination of both above conditions: if population descends one day (no matter how much), and then grows (no more then 2) - it's okay too, display this row.
• If population does not change during these 3 days, it's okay, display this row.

In addition:

• The value of population displayed in result equals to the last known value (latest date).
• The two values (+-1d) and (+-3d) are calculated.

---

I'm beginner in PHP and MySQL. I'm familiar to very basics of both, however, the MySQL part looks insane for me :( I appreciate every piece of help. And finally, sorry for my bad english. I did my best :)

Answer 1

SELECT cur.name, cur.population,
       cur.population-d1.population AS `+-1d`, cur.population-d3.population AS `+-3d`
FROM (
    SELECT name, MAX(date) AS curdate
    FROM x_world
    GROUP BY name) AS maxes
JOIN x_world AS cur ON cur.name = maxes.name AND cur.date = maxes.curdate
JOIN x_world AS d1 ON d1.name = maxes.name AND d1.date = DATE_SUB(maxes.curdate, INTERVAL 1 DAY)
JOIN x_world AS d3 ON d3.name = maxes.name AND d3.date = DATE_SUB(maxes.curdate, INTERVAL 2 DAY)
HAVING `+-1d` <= 2 AND `+-3d` <= 2

DEMO

Note that although you call it +-3d, there's actually only 2 days between 2014-05-01 and 2014-05-03. I've kept your naming, but use INTERVAL 2 DAY in the calculation.