MySQL perform subquery if count is zero - mysql

Question

I have three tables: monthly_revenue, currencies and foreign_exchange.
monthly_revenue table
|------------------------------------------------------|
| id | product_id | currency_id | value | month | year |
|------------------------------------------------------|
| 1 | 1 | 1 | 100 | 1 | 2015 |
| 2 | 1 | 2 | 125 | 1 | 2015 |
| 3 | 1 | 3 | 115 | 1 | 2015 |
| 4 | 1 | 1 | 100 | 2 | 2015 |
| 5 | 1 | 2 | 125 | 2 | 2015 |
| 6 | 1 | 3 | 115 | 2 | 2015 |
|------------------------------------------------------|
foreign_exchange table
|---------------------------------------|
| id | base | target | rate | rate_date |
|---------------------------------------|
| 1 | GBP | USD | 1.6 |2015-01-01 |
| 2 | GBP | USD | 1.62 |2015-01-15 |
| 3 | GBP | USD | 1.61 |2015-01-31 |
| 4 | EUR | USD | 1.2 |2015-01-01 |
| 5 | EUR | USD | 1.4 |2015-01-15 |
| 6 | EUR | USD | 1.4 |2015-01-31 |
| 7 | GBP | EUR | 1.4 |2015-01-01 |
| 8 | GBP | EUR | 1.45 |2015-01-15 |
| 9 | GBP | EUR | 1.44 |2015-01-31 |
|---------------------------------------|
From this, we can see the average fx rates:
GBP > USD in January is 1.61
EUR > USD in January is 1.33
GBP > EUR in January is 1.43
No rates are available for USD as a base currency, and no rates are available for February.
currencies table
|-----------|
| id | name |
|-----------|
| 1 | GBP |
| 2 | USD |
| 3 | EUR |
|-----------|
What i'm trying to achieve
Each row within the monthly_revenue table can have a different currency_id, as orders are placed is different currencies. I want to see all revenue for a given month, in a common currency. So, rather than looking at all revenue in January in GBP, and then separately looking at all revenue in January in USD, I'd like to get one value for all revenue in January - converted to USD (for example).
This can be calculated for each row, using the following (using January for this example):
revenue value x average fx rate for January between base and target currency
If I have 50 orders in January, in 4 different currencies, this let's me see all revenue in any single currency.
Example - get all revenue in January, in USD
This should return:
|------------------------------------------------------|
| id | product_id | currency_id | value | month | year |
|------------------------------------------------------|
| 1 | 1 | 1 | 100 | 1 | 2015 |
| 2 | 1 | 2 | 125 | 1 | 2015 |
| 3 | 1 | 3 | 115 | 1 | 2015 |
|------------------------------------------------------|
However, rows 1 and 3 are not in USD (these are GBP, and EUR respectively).
What I'd like to see is each row returned with the average FX rate that is being converted to, and a converted column. For example:
|-------------------------------------------------------------------------|
| id | prod_id | currency_id | value | month | year | fx_avg | converted |
|-------------------------------------------------------------------------|
| 1 | 1 | 1 | 100 | 1 | 2015 | 1.61 | 161 |
| 2 | 1 | 2 | 125 | 1 | 2015 | 1 | 125 |
| 3 | 1 | 3 | 115 | 1 | 2015 | 1.33 | 152.95 |
|-------------------------------------------------------------------------|
Where I'm at
I can currently get the basic calculation done using the query below, but a couple of key features are lacking:
If there is no FX rate available (for example for future dates where of course an FX rate isn't available) then the entire row is ignored. What I'd like in this instance is for the latest month's average to be used.
If the calculation is being performed where the target currency is the same as the base currency, the entire row is ignored (as there is no record in the FX table where the base equals the target). In this instance, the rate should be hard defined as 1.
Query so far
SELECT
r.value * IFNULL(AVG(fx.rate),1) as converted, AVG(fx.rate) as averageFx,
r.*, fx.*
FROM
foreign_exchange fx, monthly_revenue r, order_headers h
WHERE
fx.base IN (SELECT name FROM currencies WHERE id = r.currency_id) AND
r.order_header_id = h.id AND
fx.target = 'USD' AND
MONTH(fx.rate_date) = r.month AND
YEAR(fx.rate_date) = r.year AND
r.year = 2015
GROUP BY r.id
ORDER BY month ASC
If there are no records available for FX, it looks like a separate subquery should be performed to get the average of the latest month's rates.
Any input would be appreciated. If any further info is required, please post a comment.
Thanks.
Edit Here is a SQFiddle which has the example schemas and the code which highlights the issue.

Answer 1

Here is an approximation of a function that computes your exchange for a given currency and start of month:
DELIMITER //
CREATE FUNCTION MonthRate(IN _curr CHAR(3) CHARACTER SET ascii,
IN _date DATE)
RETURNS FLOAT
DETERMINISTIC
BEGIN
-- Note: _date must be the first of some month, such as '2015-02-01'
DECLARE _avg FLOAT;
DECLARE _prev FLOAT;
-- First, try to get the average for the month:
SELECT AVG(rate) INTO _avg FROM foreign_exchange
WHERE base = _curr
AND target = 'USD'
AND rate_date >= _date
AND rate_date < _date + INTERVAL 1 MONTH;
IF _avg IS NOT NULL THEN
RETURN _avg;
END;
-- Fall back onto the last rate before the month:
SELECT rate INTO _prev
FROM foreign_exchange
WHERE base = _curr
AND target = 'USD'
AND rate_date < _date
ORDER BY _date
LIMIT 1;
IF _prev IS NOT NULL THEN
RETURN _prev;
END;
SELECT "Could not get value -- ran off start of Rates table";
END;
DELIMITER ;
There are probably syntax errors, etc. But hopefully you can work with it.
It should be easy to call the function from the rest of the code.
For performance, this would be beneficial:
INDEX(base, target, rate_date, rate)

Answer 2

Create a view :
create view avg_rate as
select base, target, year(fx.rate_date) year, month(fx.rate_date) month,
avg(rate) avg_rate
from foreign_exchange group by base, target
Join it twice, once for current month, and once for previous
select r.id, r.month,
r.value * avg(coalesce(cr.avg_rate, pr.avg_rate, 1)) converted,
avg(coalesce(cr.avg_rate, pr.avg_rate), 0) rate
from monthly_revenue r, avg_rate cr, avg_rate pr, order_headers h
where
r.year = 2015 and
cr.year = r.year and cr.month = r.month and cr.target='USD' and
pr.year = r.year and pr.month = r.month - 1 and pr.target='USD' and
r.order_header_id = h.id
group by r.id
order by r.month
Also I personally don't like this way of writing query and prefer to using explicit joins as you group conditions logically and don't have a mess in where clause. i.e.:
...
from monthly_revenue r
inner join order_headers h on r.order_header_id = h.id
left join avg_rate cr on cr.year = r.year and cr.month = r.month and cr.target='USD'
left join avg_rate pr on pr.year = r.year and pr.month = r.month - 1 and pr.target='USD'
where r.year = 2015

Answer 3

http://sqlfiddle.com/#!9/6a41a/1
This fiddle based on your original one but I added some rate values for February and March to test and show how it works.
SELECT t.*,
IF(#first=t.id, #flag := #flag+1,#flag:=1) `flag`,
#first:=t.id
FROM
(SELECT
coalesce(fx.rate,1) `rate`, (r.value * coalesce(fx.rate,1)) as converted,
r.*, fx.base,fx.target, fx.avg_date, fx.rate frate
FROM
monthly_revenue r
LEFT JOIN
currencies
ON r.currency_id = currencies.id
LEFT JOIN
(SELECT AVG(rate) `rate`,
`base`,
`target`,
STR_TO_DATE(CONCAT('1/',MONTH(rate_date),'/',YEAR(rate_date)), '%d/%m/%Y') avg_date
FROM foreign_exchange
GROUP BY `base`, `target`, `avg_date`
) fx
ON currencies.name = fx.base
AND fx.target = 'USD'
AND fx.avg_date <= STR_TO_DATE(CONCAT('1/',r.month,'/',r.year), '%d/%m/%Y')
ORDER BY r.id, fx.avg_date DESC) t
HAVING `flag` = 1
and if you need records just for specific month you can add WHERE before ORDER like this:
WHERE r.month = 1 and r.year = 2015
ORDER BY r.id, fx.avg_date DESC) t

Answer 4

You may test this query on the fiddle link you provided : http://sqlfiddle.com/#!9/33def/2
select id,product_id,currency_id,currency_name,
value,month,year,#prev_fx_avg:=ifnull(fx_avg,#prev_fx_avg) fx_avg,
value*#prev_fx_avg as converted
from (SELECT
r.id,r.product_id,r.currency_id,c.name as currency_name,
r.value,r.month,r.year,if(c.name="USD",1,temp.avg_rate) as fx_avg
FROM
monthly_revenue r
left join currencies c on r.currency_id=c.id
left join
(select base , avg(rate) as avg_rate, MONTH(fx.rate_date) month,
YEAR(fx.rate_date) year
from foreign_exchange fx
where target="USD"
group by base,target,MONTH(fx.rate_date),
YEAR(fx.rate_date)) temp on(r.month=temp.month and r.year=temp.year and c.name=temp.base)
group by r.id
order by r.currency_id,r.month ASC, r.year ASC) final,(select #prev_fx_avg:=-1) temp2;