mySQL：如何遍历3个不同的表，引用每个表中的不同列，但使用主键

Question

I'm trying to find out which schools had students that did not complete their exams in 2018. So I've got 3 tables set up being: ExamInfo , ExamEntry and Students . I'm going to try use the ExamInfo table to get information from the Students table though, I obviously only want the student information that did not complete their exam in 2018. Note: I'm looking for students that attended, though did not complete the exam, with this particular exam you can look at completed exam as passed exam.

Within ExamInfo I have the columns:

ExamInfo_Date --when exam took place, using to get year() condition
ExamInfo_ExamNo --unique student exam ID used to connect with other tables
ExamInfo_Completed --1 if completed, 0 if not.
...

Within ExamEntry I have the related columns:

ExamEntry_ExamNo --connected to ExamInfo table
ExamEntry_StudentId --unique studentId used to connect to Students table
ExamEntry_Date -- this is same as ExamInfo_Date if any relevance.
...

Within Students I have following columns:

Students_Id --this is related to ExamEntry_StudentId, PRIMARY KEY
Students_School --this is the school of which I wish to be my output.
...

I want my output to simply be a list of all schools that had students that did not complete their exams in 2018. Though my issue is with getting from the ExamInfo table, to finding the schools of which students did not complete their exam.

So far I've:

SELECT a.Students_School, YEAR(l.ExamInfo_Date), l.ExamInfo_Completed
FROM ExamInfo l ??JOIN?? Students a
WHERE YEAR(l.ExamInfo_Date) = 2018
AND l.ExamInfo_Completed = 0
;

I'm not even sure if going through the ExamEntry table is necessary. I'm sure I'm meant to use a join, though unsure of how to appropriately use it. Also, with my 3 different SELECT columns, I only wish for Students_School column to be output:

Students_School
---------------
Applederry
Barnet Boys
...

Answer 1

Clearly, you need a JOIN -- two in fact. Your table has exams , students , and a junction/association table that represents the many-to-many relationship between these entities.

So, I would expect the FROM clause to look like:

FROM ExamInfo e JOIN
     ExamEntry ee
     ON ee.ExamEntry_ExamNo = e.ExamNo JOIN
     Students s
     ON ee.ExamEntry_StudentId = s.Students_Id