SQL Server 2012

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CREATE TABLE mydata
             (date datetime,
              thething varchar(4));
INSERT INTO mydata
            (date,
             thething)
            VALUES ('2016-03-09 08:17:29.867',
                    'Frob'),
                   ('2016-03-09 08:18:33.327',
                    'Frob'),
                   ('2016-03-09 14:32:01.240',
                    NULL),
                   ('2016-10-21 19:53:49.983',
                    NULL),
                   ('2016-11-12 03:25:21.753',
                    'Frob'),
                   ('2016-11-24 07:43:24.483',
                    NULL),
                   ('2016-11-28 16:06:23.090',
                    'Frob'),
                   ('2016-11-28 16:09:07.200',
                    'Frob'),
                   ('2016-12-10 11:21:55.807',
                    'Frob');
9 rows affected

SELECT date,
       thething,
       rank() OVER (ORDER BY date)
       -
       sum(CASE
             WHEN thething IS NULL THEN
               1
             ELSE
               0
           END) OVER (ORDER BY date) desiredtotal
       FROM mydata;


date
thething
desiredtotal


09/03/2016 08:17:29
Frob
1

09/03/2016 08:18:33
Frob
2

09/03/2016 14:32:01
null
2

21/10/2016 19:53:49
null
2

12/11/2016 03:25:21
Frob
3

24/11/2016 07:43:24
null
3

28/11/2016 16:06:23
Frob
4

28/11/2016 16:09:07
Frob
5

10/12/2016 11:21:55
Frob
6

date	thething	desiredtotal
09/03/2016 08:17:29	Frob	1
09/03/2016 08:18:33	Frob	2
09/03/2016 14:32:01	null	2
21/10/2016 19:53:49	null	2
12/11/2016 03:25:21	Frob	3
24/11/2016 07:43:24	null	3
28/11/2016 16:06:23	Frob	4
28/11/2016 16:09:07	Frob	5
10/12/2016 11:21:55	Frob	6