Try this
IF OBJECT_ID(N'tempdb..#TransactionTable', N'U') IS NOT NULL
DROP TABLE #TransactionTable;
create table #TransactionTable(
Department_id int not null,
date date,
comments varchar(20)
);
insert into #TransactionTable(Department_id,date,comments)
values
(1,'2/2/2018','comment 2'),
(1,'2/2/2018','comment 3'),
(1,'5/5/2017','comment 3'),
(1,'4/5/2018','comment 3'),
(1,'6/2/2018','comment 5'),
(3,'3/4/2018','comment 2'),
(3,'5/5/2017','comment 3'),
(3,'5/5/2017','comment 2'),
(3,'4/5/2018','comment 3');
select * from #TransactionTable order by Department_id,DATE,comments
SELECT Department_id,DATE,comment1 = STUFF((
SELECT distinct ',' + comments
FROM #TransactionTable t1 where t1.Department_ID= t2.Department_ID and t1.date=t2.date
FOR XML PATH ('')), 1, 1, '' )
FROM #TransactionTable t2
GROUP BY Department_ID,DATE
order by Department_id,DATE