MySQL 5.7

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CREATE TABLE test (
  col1 INT
);
INSERT INTO test VALUES
  (4), (3), (2), (1);
SELECT *
FROM test


col1


4

3

2

1


SELECT (@S:=@S+1) AS SEQ_NO, t.* 
FROM (
  SELECT col1
  FROM test 
  GROUP BY col1 
  ORDER BY col1
)t, (SELECT @S:=0) sn


SEQ_NO
col1


1
1

2
2

3
3

4
4