SQL Server: how to construct a IF like system within "where" clause?

Question

I have a table like below after several joins

username    trandate    positionname       ChannelID
--------    ----        ------------       ---------
system      01/01/2019                     1
anderson    06/04/2019  chief              1
williams    07/03/2019  chief              2
julie       15/02/2019  technician         48
julie       27/05/2019  chief technician   21

I want to count total number of transactions, sum of amount grouped by month, year and channel type.

there is a condition: if channelID is equals to "1", then query should exclude (1) transactions with has null value on "positionname" or (2) transactions with "system" username.

My code is something like this below:

    select DISTINCT

DATEPART(YEAR,a.TranDate) as [year],
DATEPART(MONTH,a.TranDate) as [month],

    count(*) as [transaction number], 
    sum(a.Amount) as [Total amount], 

    b.Name as [branch name], 
    c.ChannelName as [channel] 

    from transactions_main as a

left join branches      as b    WITH (NOLOCK) ON a.TranBranch=b.BranchId  
left join Channels      as c    WITH (NOLOCK) ON a.ChannelId=c.ChannelId  
left join Staff_Info    as d    WITH (NOLOCK) ON a.UserName=d.UserCode 

where 
        a.TranDate>'20181231' 
        and b.name not in ('HQ')
        and (a.ChannelId=1 
            and d.positionname is not null 
            and a.UserName not in ('SYSTEM', 'auto') ) 


            group by b.Name, c.ChannelName,DATEPART(YEAR,a.TranDate), DATEPART(MONTH,a.TranDate) 

order by b.Name, Year,month

In the end, of course, it didnt worked. I got the sum and count of transactions of only channelID=1's.

BONUS: You want to help another topic of mine? in which i also need help about indexing:

determining the staff's title when a certain transaction is done

here it is : link

Answer 1

use an OR operator

where 
        a.TranDate>'20181231' 
        and b.name not in ('HQ')
        and (   
                (a.ChannelId=1 
                and d.positionname is not null 
                and a.UserName not in ('SYSTEM', 'auto') ) 
                )
                OR
                a.ChannelId<>1 
            )