How to select DISTINCT rows without having the ORDER BY field selected

Question

So I have two tables students (PK sID) and mentors (PK pID). This query

SELECT s.pID
FROM students s JOIN mentors m ON s.pID = m.pID
WHERE m.tags LIKE \'%a%\         


        

Answer 1

After using distinct "ORDER BY s.sID DESC;" will not work so try using somwthing like following

SELECT distinct(s.pID) as PID
FROM students s JOIN mentors m ON s.pID = m.pID
WHERE m.tags LIKE '%a%'
ORDER BY PID;

This will return >> 3, 9, 10

Answer 2

Use this

SELECT DISTINCT s.pID as PID
FROM students s JOIN mentors m ON s.pID = m.pID
WHERE m.tags LIKE '%a%'
ORDER BY s.sID DESC,1;

Answer 3

SELECT s.pID
FROM students s JOIN mentors m ON s.pID = m.pID   
WHERE m.tags LIKE '%a%'
GROUP BY s.pID
ORDER BY MAX(s.sID) DESC

Answer 4

You can't receive records in any predefined order if you don't use ORDER BY because then the DB engine decides in what order to return them.

Answer 5

After struggling some more I have this

SELECT s.pID, MAX(s.sID) AS newest_student
FROM students s JOIN mentors m ON s.pID = m.pID
WHERE m.tags LIKE '%a%'
GROUP BY s.pID
ORDER BY newest_student DESC;

which gives me the required 9,3,10 but I have one useless field returned with it. I am hoping some-one will come with a better solution.

Answer 6

Try this:

SELECT s.pID
FROM students s JOIN mentors m ON s.pID = m.pID
WHERE m.tags LIKE '%a%'
GROUP BY s.pID
ORDER BY s.sID DESC;

I.e. GROUP BY instead of DISTINCT should preserve order.