Question

!Modified!

I have a table (mySQL 5.0.x) which contains values for date ranges.

| id | link_id | type_id | value |  start | end    |
====================================================
|  1 |       1 |       1 |    10 | 201111 | 201202 |
|  2 |       1 |       2 |    20 | 201110 | 201201 |
|  3 |       1 |       1 |   100 | 201202 | 201202 |
|  4 |       2 |       1 |    40 | 201202 | 201203 |

where

• id is the id for record
• link_id is for linking the data to other tables
• type_id is for determining the type of the value from other table
• value is numerical value
• start and end define the range (real date or integer for year and month)

The trick is that I need to present total value per type and link for a given period on a monthly basis. So the result should be like this for the range 201201 – 201202:

| period | link_id | type_id | value |
======================================
| 201201 |       1 |       1 |    10 |
| 201202 |       1 |       1 |   110 |
| 201201 |       1 |       2 |    20 |
| 201201 |       2 |    NULL |  NULL |
| 201202 |       2 |       1 |    40 |

I could use PHP to put the information like this into the database but there are some drawbacks. I have hundreds of link_ids, many type_id, and the average difference between start and end dates is 30 months so I would have tons of rows.

Answer 1

Here’s one way to do it. I’ve put in $startMonth and $endMonth so you can see how they play in. (I have this niggling feeling this can be simplified, but it currenty escapes me):

SET @MONTH:=$startMonth-1;

SELECT month, link_id, type_id, SUM(value) AS value
FROM (SELECT @MONTH:=@MONTH+1 as month
      FROM foo f
      LIMIT $endMonth-$startMonth+1) a
LEFT JOIN foo f
 ON a.month >= f.start AND a.month <= f.end
GROUP BY link_id, type_id, a.month;

The way it works: suppose you had a table a:

+-------+
| month |
+-------+
|201201 |
|201202 |
+-------+

Then the query you’re after is a lot simpler to work out:

SELECT month, link_id, type_id, SUM(value) AS value
FROM a
LEFT JOIN foo
 ON a.month >= f.start AND a.month <= f.end
GROUP BY link_id, type_id, a.month;

Ie you join table a to foo by making sure the month is between the start and end periods, and then you just sum up value, grouping by month, link id, and type id.

So the problem is how to generate table a which contains numbers from $startMonth to $endMonth. Hence the:

SET @MONTH:=201200;
SELECT @MONTH:=@MONTH+1 AS month
FROM foo
LIMIT 2;

This gives a table of numbers startign from 201201 and with 2 rows, ie up to 201202. In this query the FROM foo is dummy – not really used.