MySQL 8.0

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CREATE TABLE referralStudents (referralID int, studentID int);
INSERT INTO referralStudents (referralID, studentID) VALUES
(1, 10), (2, 10)
SELECT * FROM referralStudents


referralID
studentID


1
10

2
10


INSERT INTO referralStudents (referralID,studentID) 
SELECT 3, 30 
FROM dual
WHERE NOT EXISTS (SELECT * FROM referralStudents WHERE referralID = 3 and studentID = 30)
SELECT * FROM referralStudents


referralID
studentID


1
10

2
10

3
30


INSERT INTO referralStudents (referralID,studentID) 
SELECT 1, 10 
FROM dual
WHERE NOT EXISTS (SELECT * FROM referralStudents WHERE referralID = 1 and studentID = 10)
SELECT * FROM referralStudents


referralID
studentID


1
10

2
10

3
30