exists - What is the different between the two sql codes? -

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1. 

  Define course_id where it exists (choose course_id from course where course_id no (choose course_id from section););

2. 

  Define course_id where course_id is not (choose course_id from the section); I want to get the  course  in every  course_id  table which does not appear in the table  section  . First result in 200; Second result in 115 Why is the result of both of them different? 
 
 I first explain that I will check whether  course_id  has no  course  related to  section  if Is present, return  course_id , which is selected as  course_id

From the docs: 

  If a subquery returns a line at all, the current subcategory is true, and currently it is not subquery is FALSE

In your case, 1 query sums up all the rows It looks like 

  select course_id from the course where TRUE;

So if you want to retrieve every course_id in the table course that does not appear in the table section then your second query is correct.

And about this:

I first explain that I definitely check whether or not I'm sure that the course_id is not in that section. If present, then course_id is correct, which is the first course_id selected.

This is an incorrect interpretation:


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