Why the ID (student_id) of students who fail in my score table (num < 60) cannot use having select student_id from score having num < 60; can only use select student_id from score where num < 60 ;
mysql5.7
I want to know the principle of error reporting
having filters the returned results. There is no num in the returned results. You can filter
by adding num to the returned results. 
correctly understand the difference between where and having in MySQL https://blog.csdn.net/yexuden...
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Table structure ...
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