Merging two MySQL tables with sorted datetime

Question

Sorry if this sounds silly. Right now, I only have a basic understanding of MySQL and relational databases. Basically I can only do some simple SELECTs ;)
As usual, I did my Google and StackOverflow research before posting this, but I couldn't find any suitable answer (the ones on SO were almost every time with "0" value).
I will try to make it as generic as possible so that it could be useful to somebody else.
Let's say I have two tables:

TBUY
  qtyBuy   : integer
  dateBuy  : datetime

TSELL
  qtySell  : integer
  dateSell : datetime

I would like to be able to write a query so that I can have a resulting table of qtyBuy/qtySell SORTED by the "merge" of the two dates, e.g. if those two tables are the "history" of some items beign bought and sold, I would like to know the history of that item, sorted by date.
I know that it's fairly easy to make the two different SELECTs and then, in whatever programming language you are using, merging the result "by hand". But I was wondering if there was a better way using MySQL. I was thinking something like

SELECT * FROM (SELECT * FROM tbuy UNION ALL SELECT * FROM tsell) ORDER BY whichDate?!

But I do not know how to specify "whichDate". Thank you.

Answer 1

Have you tried:

select "buy" as action, qtyBuy as item, dateBuy as actionDate 
from TBUY
union all
select "sell" as action, qtySell as item, dateSell as actionDate
from TSELL
order by actionDate

?

Since I can't guess what is qtyBuy, I don't know if you instead should join the tables.

As soon as you want a date-line with no wholes, you need an extra table with the dates. SQL can only read and combine values, but not create them out of nothing.

Answer 2

instead of * use the names of the columns then you can rename the dateBuy and dateSell columns to something lets say date.

In the Order-part of the statement you can use the new name (dateMerged) to order the resultset.

SELECT qtyBuy as qtyChange, dateBuy as dateMerged FROM tbuy 
UNION ALL SELECT -1 * qtySell, dateSell FROM tsell
ORDER BY dateMerged

The resultset of UNION conains the columnnames of the first SELECT-statement, so you might use dateBuy too, but that is not a speaking name in the UNION...