Need help with SQL code. The only thing that I can't get is for the first line of the result to show NULL for the avgLOS.

Hospital is a database with one table, which is titled as Encounter. The table stores a series of hospital encounters by patients. Length of stay (LOS) is the difference between the discharge and admission dates of a patient (i.e., how long the patient stayed in the hospital). Note that LOS is not an attribute in the table but can be computed as said above. Write a single query that, for each record, shows a running average of LOS for all previous encounters in the database. The average must be in days (as the unit).

The output must look like this:

Here is the code that I have written so far:

create schema hospital; use hospital;

create table encounter (EncounterID int not null, PatientID int not null, DOB date, Race char(25), Gender char(1), AdmissionDate datetime, dischargeDate datetime, primary key (EncounterID));

insert into encounter values (1000000, 1, '1970-03-15', 'White', 'F', '2019-06-25 12:35:23', '2019-06-27 09:45:37'), (1000001, 2, '1955-05-04', 'Black', 'M', '2019-03-30 23:05:00', '2019-04-8 10:15:00'), (1000002, 3, '1940-11-24', 'Asian', 'F', '2018-10-12 15:45:45', '2018-10-20 13:30:35'), (1000003, 1, '1970-03-15', 'White', 'F', '2019-08-11 18:55:00', '2019-08-18 14:32:00'), (1000004, 4, '1939-07-20', 'White', 'M', '2015-02-12 20:25:10', '2015-02-28 11:40:00'), (1000005, 3, '1940-11-24', 'Asian', 'F', '2017-09-12 06:05:00', '2017-09-20 08:10:15'), (1000006, 5, '1945-12-25', 'Alaskan', 'M', '2018-11-18 06:45:05', '2018-12-01 08:40:30'), (1000007, 6, '1980-01-15', 'Black', 'M', '2019-09-25 09:33:08', '2019-09-28 08:19:35'), (1000008, 6, '1980-01-15', 'Black', 'M', '2020-02-10 16:23:24', '2020-02-14 19:58:23');

Select *, DATEDIFF(AdmissionDate,dischargeDate)AS LOS, AVG(DATEDIFF(AdmissionDate,dischargeDate)) OVER(Order BY AdmissionDate Asc ROWS 9 Preceding ) AS avgLOS FROM encounter GROUP BY EncounterID,PatientID,DOB,Race,Gender,AdmissionDate,dischargeDate Order BY AdmissionDate;

Encounter ID Patient DOB Rece Gender Admission Date dischargeDate LOS VOLOS 1000004 1939-07-20 White M 2015-02-12 20:25:10 2015-02-28 11:40:00 15.6353 1000005 3 1940-11-24 Aslan F 2017-09-12 06:05:00 2017-09-20 08:10:15 8.0870 15.63530093 1000002 3 1940-11-24 Asian F 2013-10-12 15:45:45 2013-10-20 13:30:35 7.9051 11.86114005 1000005 5 1945-12-25 Alaskan M 2018-11-18 06:45:05 2018-12-01 08:40:30 13.0602 10:54280478 1000001 2 1955-05-04 Black M 2019-03-30 23:05:00 2019-04-08 10:15:00 8.4653 11.17714120 1000000 1 1970-03-15 White F 2019-06-25 12:35:23 2019-06-27 09:45:37 1.8821 10.63476852 1000003 1 1970-03-15 White 2019-08-11 18:55:00 2019-08-18 14:32:00 6.8174 9.17599151 1000007 6 1980-01-15 Black M 2019-09-25 09:33:08 2019-09-23 08:19:35 2949 8.8390431 1000009 7 2012-08-13 White F 2020-01-14 13:45:59 2020-01-16 10:00:23 1.8434 8. 10277922 1000008 6 1980-01-15 Black M 2020-02-10 16:23:24 2020-02-14 19:58:23 4.1493 7.40729167