MySQL在另一个表中根据最近日期的价格计算值

Question

I have 2 tables where I want to calculate a value across. One table provides daily price data (prices) which is historic while the second contains annual data (volumes) which contains both historic and projected data. So far, I can join the two tables so that I can calculate the historic annual value by the price at the corresponding date in the daily one, but not for the future annual values where I would like to calculate against the last daily price in the prices table.

Current table structure

Table 1: Prices   (3 fields: date,code,price)
Table 2: Volumes  (3 fields: date,code,units)

Current query

SELECT v.date,p.price,v.units,CONCAT(p.price*v.units) AS Value
FROM Prices p,Volumes v
WHERE p.code = v.code AND p.date = v.date
AND v.code = 'X' AND (v.date BETWEEN '2012-12-31' AND '2016-12-31')
GROUP BY v.date
ORDER BY v.date
LIMIT 5;

results in 2012-2013 data rather than 2012-2016:

date   2012-12-31   2013-12-31 
price    50            58
units   100            90
Value  5000           5220

My annual table (Volumes) has volume estimates for years 2014,2015 and 2016 (which are not shown) and the last price in the daily table (Prices) is say 65, but the above query will only match the volumes to the corresponding date value in the daily price table rather than using the nearest last value. Can someone advise me on how best to do this please?

example of tables (note: both tables use a composite primary key around 'date' and 'code')

Volumes

date           code    units
2012-12-31     X        100
2012-12-31     Y         50
2013-12-31     X         90
2013-12-31     Y         45
2014-12-31     X         95
2014-12-31     Y         47

Prices

date           code    price
2013-12-31     X         50
2013-12-31     Y         25
2014-01-01     X         58
2014-01-01     Y         27
2014-01-02     X         59
2014-01-02     Y         30
-----
2014-03-31     X         48
2014-03-31     Y         26
2014-04-01     X         49
2014-04-01     Y         27
last data point

Answer 1

If you want a query to get the future values by multiplying the future projected unit quantity with the most recent price recorded then this would be the query. see working FIDDLE

SELECT 
  *, 
  (future_units*last_price) as projeted_value --- dont need the CONCAT for this calculation
FROM(
  SELECT 
    v.date, 
    v.units as future_units,
    (SELECT 
       price 
     FROM prices
     ORDER BY date desc --- desc for the most recent
     LIMIT 1) as last_price --- just need the first one
  FROM volumes v
  WHERE v.date > NOW() --- to make it a future date
)as t --- table needs alias

---

updated query per the OP request.. working FIDDLE This one calculates the price related with the date.. once you get to future dates it calculates the last price with the future units

SELECT
  *,
  if(DATE(the_date) > NOW(), --- if its a future date put in last_price, else put in the current_price
    (future_units*last_price),
    (future_units*current_price))AS value
FROM(
  SELECT
    v.date AS the_date,
    v.units AS future_units,
    v.code,
    p.price AS current_price,
    (SELECT
       price
     FROM prices
     ORDER BY date DESC
     LIMIT 1) AS last_price
  FROM prices p
  LEFT JOIN volumes v ON p.code = v.code
  WHERE p.date >= v.date OR p.date < v.date
  AND DATE(v.date) BETWEEN '2012-12-31' AND '2016-12-31' -- DATE() used to cut out timestamp for accurate comparison
  GROUP BY v.date, v.code, v.units
)AS t