Question

I have 2 tables A and B with following records

--TABLE A--  
id        date          value  
1        09/01/2012    Reward  
2        09/01/2012    Purchase  
3        07/01/2012    Reward  
4        07/01/2012    Purchase

--TABLE B--  
id   id_from_a     date           value  
1       1         10/01/2012     Generated Rewrd   
2       3         08/01/2012     Generated Reward 

Now I want the result to be like below

id        date          value  
1         10/01/2012    Generated Reward  
1         09/01/2012    Reward  
2         09/01/2012    Purchase  
3         08/01/2012    Generated Reward  
3         07/01/2012    Reward  
4         07/01/2012    Purchase  

I know using UNIONS will merge these 2 tables but how do i order it like i have mentioned above ?

Answer 1

A UNION can have a single ORDER BY clause specified at the end which applies to the entire, combined result set.

SELECT id, date, value
    FROM table_A
UNION
SELECT id, date, value
    FROM table_B
ORDER BY id, date DESC

If you don’t need the deduplicating feature of the UNION operation, a UNION ALL will yield better performance.

SELECT id, date, value
    FROM table_A
UNION ALL
SELECT id, date, value
    FROM table_B
ORDER BY id, date DESC