SQL Query Duplicates Values-CodePudding

I'm running a query to see who hasn't signed up for classes, and whenever I do it, it duplicates the student information and displays them twice. Any reason query wise it would be doing this?

Here is the query.

SELECT DISTINCT a.grade, a.lastname, a.firstname, a.sid 
FROM aspen a 
LEFT JOIN gh g ON a.sid = g.sid 
WHERE g.sid IS NULL 
order by a.grade, a.lastname, a.firstname

CodePudding user response：

Since you aren't interested in any data from gh simply use not exists

select grade, lastname, firstname, sid 
from aspen a 
where not exists (select * from gh where gh.sid = a.sid)
order by grade, lastname, firstname