Properly using the Count Function - sql

Question

In the Enrollment_Changes table, the phone model listed is the phone the subscriber changed FROM on that date.
If there is no subsequent change on Enrollment_Changes, the phone the subscriber changed TO is listed on the P_Enrollment table
For example, subscriber 12345678 enrolled on 1/5/2011 with a RAZR. On 11/1/2011 he changed FROM the RAZR. You can see what he changed TO with the next transaction on Enrollment_Changes on 05/19/2012.
How would you find the Count of subs that first enrolled with the iPhone 3?
Here is the code I have for creating the tables
Create Tables: TBL 1
USE [Test2]
GO
/****** Object: Table [dbo].[P_ENROLLMENT] ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[P_ENROLLMENT](
[Subid ] [float] NULL,
[Enrollment_Date] [datetime] NULL,
[Channel] [nvarchar](255) NULL,
[Region] [nvarchar](255) NULL,
[Active_Status] [float] NULL,
[Drop_Date] [datetime] NULL,
[Phone_Model] [nvarchar](255) NULL
) ON [PRIMARY]
GO
TBL 2
USE [Test2]
GO
/****** Object: Table [dbo].[ENROLLMENT_CHANGES] ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[ENROLLMENT_CHANGES](
[Subid] [float] NULL,
[Cdate] [datetime] NULL,
[Phone_Model] [nvarchar](255) NULL
) ON [PRIMARY]
GO
Insert TBL1
INSERT INTO [P_ENROLLMENT]([Subid ], [Enrollment_Date], [Channel], [Region], [Active_Status], [Drop_Date], [Phone_Model])
VALUES(12345678, '2011-01-05 00:00:00', 'Retail', 'Southeast', 1, NULL, 'iPhone 4');
INSERT INTO [P_ENROLLMENT]([Subid ], [Enrollment_Date], [Channel], [Region], [Active_Status], [Drop_Date], [Phone_Model])
VALUES(12346178, '2011-03-13 00:00:00', 'Indirect Dealers', 'West', 1, NULL, 'HTC Hero');
INSERT INTO [P_ENROLLMENT]([Subid ], [Enrollment_Date], [Channel], [Region], [Active_Status], [Drop_Date], [Phone_Model])
VALUES(12346679, '2011-05-19 00:00:00', 'Indirect Dealers', 'Southeast', 0, '2012-03-15 00:00:00', 'Droid 2');
INSERT INTO [P_ENROLLMENT]([Subid ], [Enrollment_Date], [Channel], [Region], [Active_Status], [Drop_Date], [Phone_Model])
VALUES(12347190, '2011-07-25 00:00:00', 'Retail', 'Northeast', 0, '2012-05-21 00:00:00', 'iPhone 4');
INSERT INTO [P_ENROLLMENT]([Subid ], [Enrollment_Date], [Channel], [Region], [Active_Status], [Drop_Date], [Phone_Model])
VALUES(12347701, '2011-08-14 00:00:00', 'Indirect Dealers', 'West', 1, NULL, 'HTC Hero');
INSERT INTO [P_ENROLLMENT]([Subid ], [Enrollment_Date], [Channel], [Region], [Active_Status], [Drop_Date], [Phone_Model])
VALUES(12348212, '2011-09-30 00:00:00', 'Retail', 'West', 1, NULL, 'Droid 2');
INSERT INTO [P_ENROLLMENT]([Subid ], [Enrollment_Date], [Channel], [Region], [Active_Status], [Drop_Date], [Phone_Model])
VALUES(12348723, '2011-10-20 00:00:00', 'Retail', 'Southeast', 1, NULL, 'Southeast');
INSERT INTO [P_ENROLLMENT]([Subid ], [Enrollment_Date], [Channel], [Region], [Active_Status], [Drop_Date], [Phone_Model])
VALUES(12349234, '2012-01-06 00:00:00', 'Indirect Dealers', 'West', 0, '2012-02-14 00:00:00', 'West');
INSERT INTO [P_ENROLLMENT]([Subid ], [Enrollment_Date], [Channel], [Region], [Active_Status], [Drop_Date], [Phone_Model])
VALUES(12349745, '2012-01-26 00:00:00', 'Retail', 'Northeast', 0, '2012-04-15 00:00:00', 'HTC Hero');
INSERT INTO [P_ENROLLMENT]([Subid ], [Enrollment_Date], [Channel], [Region], [Active_Status], [Drop_Date], [Phone_Model])
VALUES(12350256, '2012-02-11 00:00:00', 'Retail', 'Southeast', 1, NULL, 'iPhone 4');
INSERT INTO [P_ENROLLMENT]([Subid ], [Enrollment_Date], [Channel], [Region], [Active_Status], [Drop_Date], [Phone_Model])
VALUES(12350767, '2012-03-02 00:00:00', 'Indirect Dealers', 'West', 1, NULL, 'Sidekick');
INSERT INTO [P_ENROLLMENT]([Subid ], [Enrollment_Date], [Channel], [Region], [Active_Status], [Drop_Date], [Phone_Model])
VALUES(12351278, '2012-04-18 00:00:00', 'Retail', 'Midwest', 1, NULL, 'iPhone 3');
INSERT INTO [P_ENROLLMENT]([Subid ], [Enrollment_Date], [Channel], [Region], [Active_Status], [Drop_Date], [Phone_Model])
VALUES(12351789, '2012-05-08 00:00:00', 'Indirect Dealers', 'West', 0, '2012-07-04 00:00:00', 'iPhone 3');
INSERT INTO [P_ENROLLMENT]([Subid ], [Enrollment_Date], [Channel], [Region], [Active_Status], [Drop_Date], [Phone_Model])
VALUES(12352300, '2012-06-24 00:00:00', 'Retail', 'Midwest', 1, NULL, 'Droid 2');
INSERT INTO [P_ENROLLMENT]([Subid ], [Enrollment_Date], [Channel], [Region], [Active_Status], [Drop_Date], [Phone_Model])
VALUES(12352811, '2012-06-25 00:00:00', 'Retail', 'Southeast', 1, NULL, 'Sidekick');
Insert TBL2
INSERT INTO [ENROLLMENT_CHANGES]([Subid], [Cdate], [Phone_Model])
VALUES(12345678, '2011-11-01 00:00:00', 'RAZR');
INSERT INTO [ENROLLMENT_CHANGES]([Subid], [Cdate], [Phone_Model])
VALUES(12346178, '2012-01-07 00:00:00', 'HTC Hero');
INSERT INTO [ENROLLMENT_CHANGES]([Subid], [Cdate], [Phone_Model])
VALUES(12348723, '2012-01-28 00:00:00', 'RAZR');
INSERT INTO [ENROLLMENT_CHANGES]([Subid], [Cdate], [Phone_Model])
VALUES(12350256, '2012-02-21 00:00:00', 'Blackberry Bold');
INSERT INTO [ENROLLMENT_CHANGES]([Subid], [Cdate], [Phone_Model])
VALUES(12349745, '2012-05-05 00:00:00', 'HTC Hero');
INSERT INTO [ENROLLMENT_CHANGES]([Subid], [Cdate], [Phone_Model])
VALUES(12345678, '2012-05-19 00:00:00', 'Palm Pre');
INSERT INTO [ENROLLMENT_CHANGES]([Subid], [Cdate], [Phone_Model])
VALUES(12347190, '2012-05-20 00:00:00', 'HTC Hero');
INSERT INTO [ENROLLMENT_CHANGES]([Subid], [Cdate], [Phone_Model])
VALUES(12350256, '2012-05-21 00:00:00', 'Blackberry Bold');
INSERT INTO [ENROLLMENT_CHANGES]([Subid], [Cdate], [Phone_Model])
VALUES(12349234, '2012-06-04 00:00:00', 'Palm Pre');
INSERT INTO [ENROLLMENT_CHANGES]([Subid], [Cdate], [Phone_Model])
VALUES(12346178, '2012-06-05 00:00:00', 'iPhone 3');
INSERT INTO [ENROLLMENT_CHANGES]([Subid], [Cdate], [Phone_Model])
VALUES(12350767, '2012-06-10 00:00:00', 'iPhone 3');

Answer 1

For the count
select COUNT(*) Total
from
(
select e.*,
rn = row_number() over (partition by e.subid order by c.cdate desc),
first_model = coalesce(c.phone_model, e.phone_model)
from [P_ENROLLMENT] e
left join [ENROLLMENT_CHANGES] c on c.subid = e.subid
) x
where rn=1 and first_model = 'iPhone 3'
For all the records
select *
from
(
select e.*,
rn = row_number() over (partition by e.subid order by c.cdate desc),
first_model = coalesce(c.phone_model, e.phone_model)
from [P_ENROLLMENT] e
left join [ENROLLMENT_CHANGES] c on c.subid = e.subid
) x
where rn=1 and first_model = 'iPhone 3'
order by subid

Answer 2

You want to know if the first record in the table is an iPhone 3. Something like this:
select count(*)
from (select e.*,
row_number() over (partition by subid order by enrollment_date) as seqnum
from p_enrollment e
) e
where seqnum = 1 and phone_model = 'iPhone 3'

Answer 3

Perhaps I'm thinking too simply, but wouldn't either of the following do what you're looking for?:
SELECT Phone_Model
, COUNT(*) AS Initially_Enrolled
FROM p_enrollment
GROUP BY Phone_Model
(working SQLFiddle: http://sqlfiddle.com/#!3/68258/4)
or
SELECT COUNT(*) AS Initially_Enrolled
FROM p_enrollment
WHERE Phone_Model = 'iPhone 3'
(working SQLFiddle: http://sqlfiddle.com/#!3/68258/3)
Since you only want initial enrollment, the ENROLLMENT_CHANGES table is irrelevant.