MySQL的比较两个表中查找匹配的记录

Question

我试图从tbl_attn_temp获取所有数据，TraineeID是匹配tbl_assignedTraineeID或tbl_instructor_infoInstructorID。但我得到的所有数据与tbl_assignedTraineeID匹配。 tbl_instructor_info数据不或与此查询，如下表示：

tbl_attn_temp

tbl_assigned

tbl_instructor_info

SELECT t1.* 
FROM tbl_attn_temp t1, tbl_instructor_info t2, tbl_assigned t3 
WHERE t1.TraineeID=t3.TraineeID or t2.InstructorID 
GROUP by t1.traineeid, t1.attnDate 

Answer 1

Your OR condition is wrong。它应该是这样的WHERE（t1.TraineeID = t3.TraineeID OR t1.TraineeID = t2.InstructorID） 查询应该是这样的：

SELECT t1.* 
FROM tbl_attn_temp t1, tbl_instructor_info t2, tbl_assigned t3 
WHERE (t1.TraineeID = t3.TraineeID OR t1.TraineeID = t2.InstructorID) 
GROUP by t1.traineeid, t1.attnDate 

Answer 2

尝试此查询

SELECT t1.traineeid, t1.attnDate 
FROM tbl_attn_temp t1, tbl_instructor_info t2, tbl_assigned t3 
WHERE (t1.TraineeID=t3.TraineeID or t1.TraineeID=t2.InstructorID) 
GROUP by t1.traineeid, t1.attnDate 

如果使用GROUP BY子句，则可以允许按列选择组（traineeid，attnDate），并允许在一列或多列上执行集合函数。