Serial number auto generation

Facing issue in auto generation of serial number.

How can i use row_number() in proper manner or is there any alternative to achieve the expected results?

Note: I want to write query without using CASE.

SELECT 
 row_number() over (order by COALESCE(legs.sta, legs.eta) ASC) sno,
 legs.Flight_ID flightId,
 legs.flight,
 legs.flightDate           
FROM Flt_OperativeFlight_Legs LEG
ORDER BY COALESCE(legs.sta, legs.eta) ASC

Current Results:

+------+--------+-------------------------+
| sno  | flight |       flightDate        |
+------+--------+-------------------------+
|    1 | 3K0722 | 2019-01-08 17:10:00.000 |
|    2 | 3K0722 | 2019-01-08 20:20:00.000 |
|    3 | 3K0723 | 2019-01-09 17:10:00.000 |
|    4 | 3K0724 | 2019-01-10 20:20:00.000 |
+------+--------+-------------------------+

Expected Results:

+------+--------+-------------------------+
| sno  | flight |       flightDate        |
+------+--------+-------------------------+
|    1 | 3K0722 | 2019-01-08 17:10:00.000 |
|      |        | 2019-01-08 20:20:00.000 |
|    2 | 3K0723 | 2019-01-09 17:10:00.000 |
|    3 | 3K0724 | 2019-01-10 20:20:00.000 |
+------+--------+-------------------------+

Answer

You can try to use ROW_NUMBER window function in a subquery, then use DENSE_RANK window function to create no column with CASE WHEN expression get rn = 1 row.

SELECT (CASE WHEN rn = 1 THEN DENSE_RANK() over(order by flight) END) no,
       (CASE WHEN rn = 1 THEN flt_id END) flt_id,
       (CASE WHEN rn = 1 THEN flight END) flight,
       flightDate
FROM 
(
    SELECT *,ROW_NUMBER() OVER(PARTITION BY flt_id,flight ORDER BY flightDate) rn
    FROM Flt_OperativeFlight_Legs 
) t1

sqlfiddle