结合查询两个表的总和和日期

Question

I have two tables like following

challenge_table

Challenger         chdate
----------------------
abc                11-02-2012
aaa                12-02-2012
ccc                12-02-2012
bbb                13-02-2012
ddd                14-02-2012

init_table

Initiateid         indate
----------------------
a1                11-02-2012
a2                11-02-2012
a3                12-02-2012
a4                13-02-2012
a5                13-02-2012

I need a result like this

challengecount  initcount   curdate
-----------------------------------
1                2          11-02-2012
2                1          12-02-2012
1                2          13-02-2012
1                0          14-02-2012

i tried a query like this

SELECT COUNT(*) challengecount, chdate caldate FROM challenge_table
UNION ALL 
SELECT COUNT(*) Initiatecount, indate caldate FROM init_table

But it doesn't work for me.

Answer 1

Since MySQL does not support a FULL OUTER JOIN, you need to combine the 2 tables with a UNION.

Then group by the date and count the values of the 2 tables

select caldate, count(distinct Challenger), count(distinct Initiateid)
from
(
    SELECT Challenger, null as Initiateid, chdate as caldate FROM challenge_table
    UNION ALL 
    SELECT null, Initiateid, indate FROM init_table
) tmp
group by caldate