mysql - 如果列中没有结果，则获取0值-MySQL

我有以下查询

SELECT COUNT( iContactId ) AS Users, DATE( dCreatedAt ) AS ActivityDate
FROM contacts WHERE iAppId =".$iAppId."
AND DATE(dCreatedAt) IN (CURRENT_DATE(), CURRENT_DATE()-1 )
GROUP BY ActivityDate

通过这个我越来越

Users |ActivityDate
1     |2014-09-19

如果没有匹配的行，我想要0

例如

Users |ActivityDate
0     |CURRENT_DATE()
0     | CURRENT_DATE()-1

我怎样才能做到这一点。

最佳答案

你可以用工会

SELECT COUNT( iContactId ) AS Users, CURRENT_DATE() AS ActivityDate
FROM contacts WHERE iAppId =".$iAppId."
AND DATE(dCreatedAt) = CURRENT_DATE()
UNION
SELECT COUNT( iContactId ) AS Users, CURRENT_DATE() - 1 AS ActivityDate
FROM contacts WHERE iAppId =".$iAppId."
AND DATE(dCreatedAt) = CURRENT_DATE() - 1