SQL COUNT为零

Question

SELECT COUNT(*) totalStudent,
c.* 
FROM classroom c 
INNER JOIN student s
ON c.classID = s.classID 
GROUP BY c.classID 

如何在totalStudent中获得零？如果学生在教室里空了。

Answer 1

我将你的问题解释为：“有些班级没有学生。我如何包括这些？”如果这是正确的，则密钥为left join：

SELECT c.classId, COUNT(s.classId)
FROM classroom c LEFT JOIN
     student s
     ON c.classid = s.classid
GROUP BY c.classId;