Cannot get my head around this query!

Question

Ignore the coloring it was for my benefit. This query joins two tables and puts out a third table.
Basically tblContractAge is a list of ages and the buckets to which people of those ages are to be assigned. Cognos is a table of people of certain ages. This is a vast oversimplification but will suffice.
The customer is expecting the resulting table to contain rows even if no people (in Cognos) table have the age for the bucket (in tblContract), This is not happening. No rows are output in this case. I'm confused as to the effect of the DISTINCT and GROUP
BY and how this works combined with the ON and WHERE. As I understand it, the ON does the join and the WHERE filters the joined tables. But when does the DISTINCT and GROUP BY come into effect? Can anyone clarify this?
SELECT DISTINCT tblContractAge.Seq AS [order], tblContractAge.Seq, tblContractAge.CMS, Cognos.Primary_Ahcccs_Id AS [Primary Ahcccs Id], Cognos.Title, tblContractAge.Age_range, tblContractAge.Service_Delivery, IIf([cognos].[title]='Title XXI',0,IIf([FPL_Percent]='0
- 100',1,0)) AS [0-100], IIf([cognos].[title]='Title XXI',1,IIf([FPL_Percent]='101 - 200',1,0)) AS [101-200], 1 AS Ever_Enrolled, Cognos.New9 AS New, Cognos.Disenrolled, IIf([Disenrolled]=1,0,1) AS EligAtEnd, Cognos.Member_Months, Cognos.Sex, Cognos.Race,
Cognos.Ethnicity INTO mtblDistribute
FROM tblContractAge LEFT JOIN Cognos ON (tblContractAge.[Contract Type] = Cognos.[Contract_Type]) AND (tblContractAge.Title = Cognos.Title)
WHERE (((IIf([cognos].[age]-Fix([cognos].[age])>0.999,Round([cognos].[age],0),Fix([cognos].[age])))=[tblContractAge].[age])) OR ((Cognos.Title='Title XXI adult'))
GROUP BY tblContractAge.Seq, tblContractAge.CMS, Cognos.Primary_Ahcccs_Id, Cognos.Title, tblContractAge.Age_range, tblContractAge.Service_Delivery, IIf([cognos].[title]='Title XXI',0,IIf([FPL_Percent]='0 - 100',1,0)), IIf([cognos].[title]='Title
XXI',1,IIf([FPL_Percent]='101 - 200',1,0)), 1, Cognos.New9, Cognos.Disenrolled, IIf([Disenrolled]=1,0,1), Cognos.Member_Months, Cognos.Sex, Cognos.Race, Cognos.Ethnicity, tblContractAge.Seq;

Answer 1

 As I understand it, the ON does the join and the WHERE filters the joined tables. But when does the DISTINCT and GROUP BY come into effect? Can anyone clarify this?
DISTINCT is to output a single record when there are multiple identical ones.
GROUP BY is the roll up level when doing SUM, MIN, MAX, AVG, COUNT, etc.
I do not understand what you are doing with the [Cognos].[age] in the WHERE.   Do you have decimal ages?
Build a little, test a little