Question

I’ve been trying to find a solution to this challenge all day.

I’ve got a table:

  id   | amount  | type |            date            |              description              | club_id 
-------+---------+------+----------------------------+---------------------------------------+---------+--------
   783 |   10000 |    5 | 2011-08-23 12:52:19.995249 | Sign on fee                           |       7 

The table has a lot more data than this.

What I’m trying to do is get the sum of amount for each week, given a specific club_id.

The last thing I ended up with was this, but it doesn’t work:

WITH RECURSIVE t AS (
    SELECT EXTRACT(WEEK FROM date) AS week, amount FROM club_expenses WHERE club_id = 20 AND EXTRACT(WEEK FROM date) < 10 ORDER BY week 
    UNION ALL 
    SELECT week+1, amount FROM t WHERE week < 3
)
SELECT week, amount FROM t;

I’m not sure why it doesn’t work, but it complains about the UNION ALL.

I’ll be off to bed in a minute, so I won’t be able to see any answers before tomorrow (sorry).

I hope I’ve described it adequately.

Thanks in advance!

Answer 1

It looks to me like you are trying to use the UNION ALL to retrieve a subset of the first part of the query. That won’t work. You have two options. The first is to use user defined functions to add behavior as you need it and the second is to nest your WITH clauses. I tend to prefer the former, but you may be preferring the latter.

To do the functions/table methods approach you create a function which accepts as input a row from a table and does not hit the table directly. This provides a bunch of benefits including the ability to easily index the output. Here the function would look like:

CREATE FUNCTION week(club_expense) RETURNS int LANGUAGE SQL IMMUTABLE AS $$
    select EXTRACT(WEEK FROM $1.date)
$$;

Now you have a usable macro which can be used where you would use a column. You can then:

SELECT c.week, sum(amount) FROM club_expense c
 GROUP BY c.week;

Note that the c. before week is not optional. The parser converts that into week(c). If you want to limit this to a year, you can do the same with years.

This is a really neat, useful feature of Postgres.