Can understand the meaning of these two not exists, but don't understand the last where Sno=Student. Sno and Cno=Course. Cno is how to query all the courses in the Course of, in figure 2, said: at this point, each Course can not output, the Student number corresponding tuples can be output, concrete is how to implement this process?

CodePudding user response:

This if NOT understand, to understand the above change is NOT IN, try to understand,

CodePudding user response:

How specific executive who know, writing this book was just assume a possibility out you just understand a layer not exist