SQL为嵌套选择创建别名列？

Question

Say I have a Person table and a Courses table. In the Person table I have the column PersonName . In the Courses table, let's say I have CourseTitle , PersonName and CourseDifficulty . CourseDifficulty is 1-4 (4 being the hardest). How do I return a list of people from Person and for each person have a column that shows the most difficult class they're taking by CourseTitle .

As far as I know, I'd get the CourseTitle of the most difficult class Brett is taking by doing the following:

SELECT CourseTitle 
FROM Courses 
WHERE PersonName = 'Brett' 
  AND CourseDifficulty = (SELECT MAX(CourseDifficulty) 
                          FROM Courses 
                          WHERE PersonName='Brett')

But how do I run that for each person in the Person table? I want the results to be something like

Brett-SQL For Dummies 4
Tim-Quantum Mechanics
Jane-Thermodynamics 2

Sorry for the noobness. Thanks in advance for the help!

Answer 1

you can use the following

SELECT p.name ,p.address, c.courseTitle ,c.courseDifficulty FROM (
        SELECT personName, courseTitle, MAX(courseDifficulty) AS courseDifficulty
        FROM course
        GROUP BY personName 
) AS c RIGHT JOIN person AS p ON p.name = c.personName

here am assuming personName is a unique. Otherwise you can use unique id over here instead on person name and add this field in select statement.

Answer 2

Try this:

SELECT c.CourseTitle, c.PersonName, c.CourseDifficulty
  FROM Courses c
 WHERE c.CourseDifficulty=(SELECT MAX(c2.CourseDifficulty) FROM Courses c2 WHERE c2.PersonName=c.PersonName)