SQL Server 2017

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CREATE TABLE mytable(
   PID           INTEGER  NOT NULL
  ,AppNo         INTEGER  
  ,ProcessedDate VARCHAR(21) NOT NULL
);
INSERT INTO mytable(PID,ProcessedDate) VALUES (11,'09/30/2019 18:21');
INSERT INTO mytable(PID,ProcessedDate) VALUES (3,'09/25/2019 08:37');
INSERT INTO mytable(PID,ProcessedDate) VALUES (3,'09/25/2019 08:37');
INSERT INTO mytable(PID,ProcessedDate) VALUES (11,'09/25/2019 08:39');
INSERT INTO mytable(PID,ProcessedDate) VALUES (11,'09/25/2019 08:40');
INSERT INTO mytable(PID,ProcessedDate) VALUES (7,'09/26/2019 14:19');
INSERT INTO mytable(PID,ProcessedDate) VALUES (7,'09/26/2019 14:20');
INSERT INTO mytable(PID,ProcessedDate) VALUES (7,'09/26/2019 14:22');
INSERT INTO mytable(PID,ProcessedDate) VALUES (2,'09/26/2019 14:23');
INSERT INTO mytable(PID,ProcessedDate) VALUES (11,'09/26/2019 14:23');
INSERT INTO mytable(PID,ProcessedDate) VALUES (11,'09/26/2019 14:24');
INSERT INTO mytable(PID,ProcessedDate) VALUES (11,'09/26/2019 14:24');
INSERT INTO mytable(PID,ProcessedDate) VALUES (3,'09/26/2019 14:24');
13 rows affected

SELECT * FROM mytable


PID
AppNo
ProcessedDate


11
null
09/30/2019 18:21

3
null
09/25/2019 08:37

3
null
09/25/2019 08:37

11
null
09/25/2019 08:39

11
null
09/25/2019 08:40

7
null
09/26/2019 14:19

7
null
09/26/2019 14:20

7
null
09/26/2019 14:22

2
null
09/26/2019 14:23

11
null
09/26/2019 14:23

11
null
09/26/2019 14:24

11
null
09/26/2019 14:24

3
null
09/26/2019 14:24


WITH cte AS (
    SELECT 
        pid, 
        ProcessedDate, 
        AppNo, 
        ROW_NUMBER() OVER(PARTITION BY pid, CAST(ProcessedDate AS DATE) ORDER BY ProcessedDate) rn 
    FROM mytable
)
UPDATE cte SET AppNo = rn
13 rows affected

SELECT * FROM mytable


PID
AppNo
ProcessedDate


11
1
09/30/2019 18:21

3
1
09/25/2019 08:37

3
2
09/25/2019 08:37

11
1
09/25/2019 08:39

11
2
09/25/2019 08:40

7
1
09/26/2019 14:19

7
2
09/26/2019 14:20

7
3
09/26/2019 14:22

2
1
09/26/2019 14:23

11
1
09/26/2019 14:23

11
2
09/26/2019 14:24

11
3
09/26/2019 14:24

3
1
09/26/2019 14:24

PID	AppNo	ProcessedDate
11	null	09/30/2019 18:21
3	null	09/25/2019 08:37
3	null	09/25/2019 08:37
11	null	09/25/2019 08:39
11	null	09/25/2019 08:40
7	null	09/26/2019 14:19
7	null	09/26/2019 14:20
7	null	09/26/2019 14:22
2	null	09/26/2019 14:23
11	null	09/26/2019 14:23
11	null	09/26/2019 14:24
11	null	09/26/2019 14:24
3	null	09/26/2019 14:24