Retrieve and compute prices in MySQL using conditions - mysql

Question

I want to retrieve/compute the price on a given date for different assets, depending on the side of the transaction. Prior to 2000, I have mid quotes, afterwards I have bid and ask or offer quotes, so I would like the price to be the average of these two quotes. More precisely:
SELECT date, price,
CASE WHEN side='' THEN 'price_mid'
WHEN side='A' THEN 'price_ask'
WHEN side='B' THEN 'price_bid'
WHEN side='O' THEN 'price_offer'
END as prices
FROM table
WHERE asset = 'a';
How can I then compute the price in a new column, having the price_mid (prior to 2000) and (price_bid+price_ask)/2 or (price_bid+price_offer)/2 afterwards?
E.g.: Let's say I have the following situation:
date price prices
1 1 price_mid
2 1.1 price_mid
3 0.9 price_bid
3 1.2 price_ask
4 1.3 price_offer
4 1.1 price_bid
And I would like to have:
date final_price
1 1
2 1.1
3 1.05
4 1.2

Answer 1

I understand you need the average for only some dates. Maybe the following does what you want:
SELECT date, AVG(price) as AvgValue
FROM prices
WHERE date >= 2
AND prices in ('price_ask','price_offer','price_bid')
GROUP BY date
UNION
SELECT date, price as AvgValue
FROM prices
WHERE date < 2
AND prices = 'price_mid'
GROUP BY date
UNION
SELECT date, price as AvgValue
FROM prices p
WHERE date >= 2
AND prices = 'price_mid'
AND NOT EXISTS (SELECT 1 FROM prices p2 where p2.date = p.date AND p2.prices in ('price_ask','price_offer','price_bid'))
GROUP BY date
ORDER BY DATE ASC

Answer 2

Something like this
SELECT `date`, AVG(price) AS final_price FROM tbl GROUP BY 1

Answer 3

Tried something, it did not work, nevertheless maybe that helps..
This is a changed version of the query you already have, but it does not save a kind of state, it only has all the values on the right places.
SELECT
date,
CASE WHEN side = '' THEN price
ELSE NULL
END as price_mid,
CASE WHEN side = 'A' THEN price
ELSE NULL
END as price_ask,
CASE WHEN side = 'B' THEN price
ELSE NULL
END as price_bid,
CASE WHEN side = '' THEN price
ELSE NULL
END as price_offer
FROM
table
WHERE
asset = 'a';
What should give you:
date price_mid price_ask price_bid price_offer
1 1 NULL NULL NULL
2 1.1 NULL NULL NULL
3 NULL NULL 0.9 NULL
3 NULL 1.2 NULL NULL
4 NULL NULL NULL 1,4
4 NULL NULL 1.1 NULL
Now on that table you can check with IS NULL or take the SUM()or the MAX() per date, so you can switch through your cases on one line.