SELECT MAX() contained within a DATEDIFF() - asp.net

Question

I am writing a report for the desktop support team in the company where I work. The report needs to produce a set of new starters within a specified time frame passed in from an ASP.NET application. Currently there is a one to many relationship between our Worker table and Contract table. We hire a lot of contractors and they sometimes come back after a number of months but are still treated like new starters as new machines need to be configured along with desk space.
A new contract is added for every pay review, job title change and new starter. We need to filter out all but the new starter. The newest contract that is added for job changes and pay reviews is always one day after the end date of the previous contract naturally. As I am only still a fresher in the grand scheme of things I am struggling with a set of functions I am trying to use to achieve my goal.
WHERE
(dbo.[Contract].StartDate BETWEEN #StartDateF AND #EndDateF) AND DATEDIFF(day, SELECT MAX(StartDate)FROM dbo.[Contract] WHERE dbo.[Contract].Worker_ID = w1.Worker_ID, SELECT MAX(EndDate)FROM dbo.[Contract] WHERE dbo.[Contract].Worker_ID = w1.Worker_ID)> 1
I basically want to find out in the instance an employee has more than one contract, regardless of leaving and coming back or pay review, if the current active contract is one day different to the previous contract. This should by my thinking give me all new starters only.
Trouble is I am still trying to get my head around when to use aggregate functions not in a select and when to apply the HAVING clause.
Any help would be appreciated to help me understand why my lack of understanding is causing this query/logic to fail.
Thanks
EDIT
Ok I am still bashing away at this solution and this is syntactically incorrect. In an attempt to remove some of the ambiguity here is the query, with an update;
Declare #StartDateF varchar(10)
Set #StartDateF = '2012-08-03'
Declare #EndDateF varchar(10)
Set #EndDateF = '2012-09-04'
SELECT w1.Worker_ID, w1.Title, w1.FirstName, w1.Surname,w1.Gender, w1.DateofBirth,
dbo.[Contract].StartDate, (select w2.surname + ',' + w2.firstname from worker w2 WITH (NOLOCK) where w2.worker_ID = w1.manager)as Manager, dbo.Grade.GradeDescription AS JobTitle, dbo.Grade.Discipline,
CASE WHEN dbo.[Contract].ContractType_ID = 1 OR dbo.[Contract].ContractType_ID = 2 OR dbo.[Contract].ContractType_ID = 5 OR dbo.[Contract].ContractType_ID = 6
THEN 'Staff' ELSE 'Contractor' END AS ContractType
FROM dbo.Worker w1 WITH (NOLOCK) inner join
dbo.[Contract] WITH (NOLOCK) ON dbo.[Contract].Worker_ID = w1.Worker_ID inner join
dbo.Grade WITH (NOLOCK) ON dbo.Grade.Grade_ID = dbo.[Contract].Grade_ID
WHERE
(dbo.[Contract].StartDate BETWEEN #StartDateF AND #EndDateF AND EndDate IS NULL)
group by
w1.Worker_ID, w1.Title, w1.FirstName, w1.Surname,w1.Gender, w1.DateofBirth,
dbo.[Contract].StartDate, manager, dbo.Grade.Discipline,dbo.Grade.GradeDescription, dbo.[Contract].ContractType_ID
Having DATEDIFF(day, SELECT MAX(StartDate)FROM dbo.[Contract] WHERE dbo.[Contract].Worker_ID = w1.Worker_ID, SELECT MAX(EndDate)FROM dbo.[Contract] WHERE dbo.[Contract].Worker_ID = w1.Worker_ID)
I have added the group by and the having clause but now I am getting the following errors
Msg 156, Level 15, State 1, Line 24
Incorrect syntax near the keyword 'SELECT'.
Msg 102, Level 15, State 1, Line 24
Incorrect syntax near ','.
Msg 102, Level 15, State 1, Line 24
Incorrect syntax near ')'.
These all relate the the functions in the having clause no doubt you can see. But I cannot understand what is wrong with this query and this is mainly the question. I need to understand the SQL functions enough so that I can implement th correct solution.
I have followed up the DATEDIFF() function here http://msdn.microsoft.com/en-us/library/ms189794.aspx
I can see that using functions within this function is acceptable according to the MS documentation.
EDIT
Commenting out the Having clause gives me the result set I expect. It is showing people with changes to contracts(pay rise) but this is information that no one should be seeing, these are now the only records that need filtering out
EDIT
I have made some improvements and overcome the error messages now, but I am still getting people where pay rises have occured. Here is the amended query from the group by
group by
w1.Worker_ID, w1.Title, w1.FirstName, w1.Surname,w1.Gender, w1.DateofBirth,
dbo.[Contract].StartDate, manager, dbo.Grade.Discipline,dbo.Grade.GradeDescription, dbo.[Contract].ContractType_ID, w1.Worker_ID
Having
(((dbo.[Contract].StartDate BETWEEN #StartDateF AND #EndDateF)
AND COUNT(dbo.[Contract].Worker_ID) = 1)
OR
((dbo.[Contract].StartDate BETWEEN #StartDateF AND #EndDateF)
AND DATEDIFF(day, (SELECT MAX(EndDate)FROM dbo.[Contract] WHERE dbo.[Contract].Worker_ID = w1.Worker_ID), (SELECT MAX(StartDate)FROM dbo.[Contract] WHERE dbo.[Contract].Worker_ID = w1.Worker_ID))>1))

Answer 1

To get workers with more than one contract, you would use:
select c.workerID
from Contract c
group by c.workerID
having count(distinct contractID) > 1
It sounds, though, like you only want to count everything but the new start ones. You can do this with something like:
select w.workerID
from Contract c
where c.ContractType = 'New'
group by w.workerID
having count(distinct contractID) > 1
Because you didn't provide the details of what the tables look like, what sample input data looks like, and the results you want to achieve, this is about the best that can be done.

Answer 2

WHERE ( (dbo.[Contract].StartDate BETWEEN #StartDateF AND #EndDateF)AND dbo.[Contract].Worker_ID
IN (select worker_id from dbo.[Contract]
group by worker_id
having count(worker_id) = 1))
OR
((dbo.[Contract].StartDate BETWEEN #StartDateF AND #EndDateF)
AND DATEDIFF(day, (SELECT MAX(EndDate)FROM dbo.[Contract] WHERE dbo.[Contract].Worker_ID = w1.Worker_ID), (SELECT MAX(StartDate)FROM dbo.[Contract] WHERE dbo.[Contract].Worker_ID = w1.Worker_ID))>1
AND dbo.[Contract].Worker_ID = w1.Worker_ID )
Now works for me :)