MySQL 5.6

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SELECT VERSION();


VERSION()


5.6.45


DROP TABLE IF EXISTS `user_course`;

CREATE TABLE IF NOT EXISTS `user_course` (
  `USER_ID` VARCHAR(16),
  `COURSE_ID` VARCHAR(16),
  INDEX(`USER_ID`, `COURSE_ID`)
);
INSERT INTO
  `user_course`
VALUES
  ('USERID1', 'COURSEID1'),
  ('USERID1', 'COURSEID1');
SELECT `USER_ID`, `COURSE_ID`
FROM `user_course`;


USER_ID
COURSE_ID


USERID1
COURSEID1

USERID1
COURSEID1


DROP TABLE IF EXISTS `user_course`;

CREATE TABLE IF NOT EXISTS `user_course` (
  `USER_ID` VARCHAR(16),
  `COURSE_ID` VARCHAR(16),
  UNIQUE INDEX(`USER_ID`, `COURSE_ID`)
);
INSERT INTO
  `user_course`
VALUES
  ('USERID1', 'COURSEID1'),
  ('USERID1', 'COURSEID1');
Duplicate entry 'USERID1-COURSEID1' for key 'USER_ID'