在MySQL中我可以在另一个表中将id FK与varchar进行比较

Question

I have table with courseID, studentName and the other table is courseID , courseName user will search for the course name in the student table based on the student name what sql syntax should I use ? join or inner join

Answer 1

You can try this:

select a.studentName,b.courseName 
from tbl_name a 
inner join tbl_name b
on a.courseID = b.courseID 

Answer 2

If you use JOIN and specify a constraint (eg on a.courseId = b.courseId ) it is exactly the same as if you use INNER JOIN , so according to your table structure you should do something like:

select * from studentTable
inner join courseTable 
on studentTable.courseID = courseTable.courseID
where studentTable.studentName = 'Jack Black';

In MySQL you could also write

select * from studentTable, courseTable 
where studentTable.courseID = courseTable.courseID
and studentName = 'Jack Black';

as it would internally be queried in the same way.

Check out the exact syntax for joins here: http://dev.mysql.com/doc/refman/5.7/en/join.html

Answer 3

You join when you are looking for a joined result, such as select all student names along with all their courses. Eg:

select s.studentname, c.coursename
from course c
join student_takes_course s on s.courseid = c.courseid;

or (as the order of the tables doesn't matter):

select s.studentname, c.coursename
from student_takes_course s
join course c on s.courseid = c.courseid;

JOIN is just short for INNER JOIN . You can use either.

But when you are not interested in the joined result, then it's a good habit not to join. Such as when you simply want to show the course names from the course table, as in your example. You would then use IN or EXISTS usually:

select coursename 
from course
where courseid in 
(
  select courseid
  from student_takes_course
  where studentname = 'Joe'
);

or

select c.coursename 
from course c
where exists
(
  select *
  from student_takes_course s
  where s.studentname = 'Joe'
  and s.courseid = c.courseid
);

(I prefer IN clauses over EXISTS clauses for their simplicity and use them when possible.)