我有以下选择语句,我需要对表 tbTasks 中的每个任务求和,并按表 tbProjects 中的 projectId 对它们进行分组,以便获得如下记录:
ProjectID = 1, ProjectName = 'My Project', TotalTime = 300 //<--sum of each task time
查询如下所示:
SELECT tbTasks.projectId,
SUM(tbTasks.taskTime) AS totalTime,
tbProjects.projectName
FROM tbTasks
INNER JOIN tbProjects ON tbTasks.projectId = tbProjects.projectId
GROUP BY tbTasks.projectId
ORDER BY tbProjects.created DESC
这可以正常工作并且执行得很好,但是有一个问题,如果一个项目没有与之关联的任务,那么我根本不会得到任何记录(我想获取 projectId、projectName 和 0 或 NULL 的 totalTime)。因此,为了正确加入表 tbProjects SQLite3 迫使我以迂回的方式进行。
SELECT tbTasks.projectId,
SUM(tbTasks.taskTime) AS totalTime,
tbProjects.projectName
FROM tbTasks LEFT OUTER JOIN tbProjects
ON tbTasks.projectId = tbProjects.projectId
GROUP BY tbTasks.projectId
UNION
SELECT tbProjects.projectId,
SUM(tbTasks.taskTime) AS totalTime,
tbProjects.projectName
FROM tbProjects LEFT OUTER JOIN tbTasks
ON tbProjects.projectId = tbTasks.projectId
GROUP BY tbTasks.projectId
ORDER BY tbProjects.created DESC
只有这不起作用,我得到一个 SQL 语法错误。我究竟做错了什么?有没有更好的方法来实现我的目标?