放在外面：
SELECt COALESCE(( SELECT SUM(i.Logged) FROM tbl_Sites s INNER JOIN tbl_Incidents i ON s.Location = i.Locatio餃﹏ WHERe s.Sites = @SiteName AND i.[month] = DATEADD(mm, DATEDIFF(mm, 0, GetDate()) -1,0) GROUP BY s.Sites ), 0) AS LoggedIncidents
如果要返回多行，请将INNER JOIN更改为 LEFT JOIN
SELECt COALESCE(SUM(i.Logged),0)FROM tbl_Sites s LEFT JOIN tbl_Incidents i ON s.Location = i.Locatio餃﹏ WHERe s.Sites = @SiteName AND i.[month] = DATEADD(mm, DATEDIFF(mm, 0, GetDate()) -1,0) GROUP BY s.Sites
顺便说一句，如果没有保证，请勿在聚合函数中放置任何函数或表达式，例如，不要在ISUM中放置ISNULL，COALESCE，在聚合内部使用函数/表达式削弱性能，查询将通过表扫描执行
