compare values of cells in MySQL

Question

I have a database as:

Student (ID, Name, Grade)
Likes (ID1, ID2)

Where ID1 and ID2 in last table are foreign key referenced student(ID)

Note: Liking isn't a mutual relation, eg its not necessary that if (123, 456) is in Likes table, then (456,123) is also in Likes table.

I have to write query for the following statement: "For every pair of students, who both like each other, return the name and grade of both students. Include each pair only once, with the two names in alphabetical order."

So far I have given the data in which ID1 and ID2 mutually like each other:

SELECT s1.ID, s1.name, s2.ID, s2.name 
FROM student s1, student s2, likes l
WHERE s1.ID = l.ID1 AND s2.ID = l.ID2 
AND l.ID1 IN (SELECT ID2 FROM likes) 
AND l.ID2 IN (SELECT ID1 FROM likes);

Someone kindly help me how to avoid duplicate pairs.

Database is: (If someone needs it)

INSERT INTO `student` VALUES (1025,'John',12),(1101,'Haley',10),(1247,'Alexis',11),(1304,'Jordan',12),(1316,'Austin',11),(1381,'Tiffany',9),(1468,'Kris',10),(1501,'Jessica',11),(1510,'Jordan',9),(1641,'Brittany',10),(1661,'Logan',12),(1689,'Gabriel',9),(1709,'Cassandra',9),(1782,'Andrew',10),(1911,'Gabriel',11),(1934,'Kyle',12);
INSERT INTO `likes` VALUES (1689,1709),(1709,1689),(1782,1709),(1911,1247),(1247,1468),(1641,1468),(1316,1304),(1501,1934),(1934,1501),(1025,1101);

and according to data entered:

DATA I GET

1689    Gabriel     1709    Cassandra
1709    Cassandra   1689    Gabriel
1501    Jessica     1934    Kyle
1934    Kyle        1501    Jessica

IDEAL DATA

1689    Gabriel     1709    Cassandra
1501    Jessica     1934    Kyle

Answer 1

Since the question is " how to avoid duplicate pairs. ":

You join 2 tables to get the ones where they both like each other, you will get 2 rows for each pair.

You can discard one by comparing against some distinct value. ID is a great candidate:

select * -- put fields here
from likes li 
join likes li2 on li2.ID1 = li.ID2 and li2.ID2 = li.ID1
-- join 2 students here
where li.ID1 < li.ID2

Answer 2

Try below query:

SELECT L1.*
FROM `likes` l1
LEFT JOIN `likes` l2 ON l1.id1 = l2.id2 AND l1.id2 = l2.id1
WHERE l2.id2 IS NOT NULL
GROUP BY l1.id1 - l1.id2
HAVING l1.id1 - l1.id2 < 0