combine two tables and add up results

Question

I have 2 tables with partially same columns which I want to combine in one query:

table 1
|id    |user  |amount |date
|1     |4     |1000   |2014-01-01 01:00:00
|2     |1     |2000   |2014-01-01 01:00:00
|3     |2     |1500   |2014-01-01 01:00:00

table 2
|id    |user  |amount |date                |flag
|1     |4     |1000   |2014-01-01 01:00:00 |0
|2     |3     |2000   |2014-01-01 01:00:00 |1
|3     |5     |1500   |2014-01-01 01:00:00 |2

I want to list the contents the two tables have in common but I want to add the amount column. So if a user is in table 1 and 2, even multiple times in both it should just list him 1 time in the result but the amount should add up.

So a result should be the following:

result
|user  |amount |date                
|4     |2000   |2014-01-01 01:00:00  <- note the 2000 here (1000+1000)
|1     |2000   |2014-01-01 01:00:00 
|2     |1500   |2014-01-01 01:00:00 
|3     |2000   |2014-01-01 01:00:00 
|5     |1500   |2014-01-01 01:00:00 

I thought of a query like

SELECT user, amount, date FROM (
    (SELECT user, amount, date FROM table1)
UNION
    (SELECT user, amount, date FROM table2)
) t1

but this doesn't add up the amount value and entries get listed multiple times. If I use GROUP BY, it only lists entries 1 time but entries get cut meaning amount also doesn't sum up. Using SUM, it adds just all entries and displays 1 entry but I need to have all entries 1 time with the amount column sum up.

Answer 1

Try this using aggregate functions you should use group by without group by you will get only one row

SELECT t1.user, SUM(t1.amount), t1.date FROM (
    (SELECT user, amount, date FROM table1)
UNION ALL
    (SELECT user, amount, date FROM table2)
) t1

GROUP BY t1.user

GROUP BY (Aggregate) Functions