Records exist or not if exist need need updated based on condition in sql server

Hi I have doubt in sql server

Trantable:
empid | deptid | Projectname | Transactionid
1 |10 | test | 1
2 |11 | test1 | 2
2 |10 | jai | 3
2nd table: dimemp ....> here dimemp is scdtype2 dimension.its all ready done

empkey | empid | ename | flag
1 | 1 | a | 1
2 | 2 | b | 1
3 | -1 | na | 1
3rd table: dimdept------>here dimdept is scdtype2 dimension.implementaion allready done.

deptkey | deptid | deptname | flag
1 | 10 | hr | 1
2 | 11 | ceo | 1
3 | -1 | NA | 1
Here I want load trantable data into facttran table with corresponding keys. here transactionid is unique column to identiy unique record.

Facttran table structure look like below and factran we need to maintain scd type1 data.

empkey | deptkey | projectname |transactionid
I tried like below query

merge into facttran target
using (select ISNULL(a.empkey, (select empkey from Dimemp where empid = -1)) empkey,ISNULL(b.deptkey, (select deptkey from dimdept where deptid = -1)) deptkey, c.projectname, c.transactionid
from trantable c
left join dimemp a on a.empid=c.empid and a.flag=1
left join dimdept b on b.deptid=c.deptid and b.flag=1)source
on target.transactionid=source.transactionid
when not matched
then insert ([deptkey],[empkey],[projectname],[transactionid])
values(source.deptkey,source.empkey,source.projectname,source.transactionid)
when matched
then update set target.empkey=source.empkey ,
target.deptkey=source.deptkey,
target.projectname=source.projectname,
target.transactionid=source.transactionid ;
then I got output like below

Table :facttran
empkey | deptkey | projectname |transactionid
1 | 1 | test | 1
2 | 2 | test1 | 2
2 | 1 | Jai | 3
upto now its working fine.

2nd day in my trantable few records updated and few records insert in sourc trantable.based on below table data I want update in facttable with corresponding key. 2nd table: dimemp ....> here dimemp is scdtype2 dimension

empkey | empid | ename | flag
1 | 10 | a | 0
2 | 11 | b | 1
3 | -1 | na | 1
4 | 10 | aaa | 1
3rd table: dimdept------>here dimdept is scdtype2 dimension.implementaion allready done.

deptkey | deptid | deptname | flag
1 | 10 | hr | 0
2 | 20 | ceo | 1
3 | -1 | NA | 1
4 | 10 |hrdept | 1
Trantable:

empid | deptid | Projectname | Transactionid
1 |11 | test | 1 ------record updated in source side here deptid changed from 10 to 11
1 |11 | test123 | 2 -------Here empid changed from empid 2 to 1 and projectname changed test1 to test123
2 |10 | jai | 3 ------here no records are not changed
1 |10 | cod | 4 ----------new rocrd is came
based on above trantable.I want facttran table data look like below.

Table :facttran
empkey | deptkey | projectname |transactionid
1 | 2 | test | 1
1 | 2 | test123 | 2
2 | 1 | Jai | 3
4 | 10 | cod | 4
when I ran 2nd time with same query.I am not able get to expected result. here mainily source trantable related transactioni id is exist or not in facttran table .if not exist then we need to insert correspondig dimensionkeys with lates flaf=1 values.if we found transactionid exist in fact table then we need to updated existing dimension corresonding key. suppose if we take transactionid=1 records here only chnaged deptid not empid that time we donot need update empid corresponding lates flag=1 corresondingkey we need check exisig transaction id is updated each dimension need to check exist or not if not exist latest flag=1 related corresponding key. if exist we donot need to updated that one.if new reocrds came then we need to insert with latest flag=1 corresponding keys in factran table.

please tell me how to write query to achive this task in sql server.