How merger works in Sql Server in Database

-------Merge----
--select * from city

--now lets try to insert a new id...Let's do it by stored procedure

alter procedure insertproc
@id int,
@name varchar(70)

as
begin

update city set name=@name where id=@id

if(@@rowcount=0)
begin

insert into city(id,name) values(@id,@name)

end

end

--exec insertproc 6,'calcutta'
--select * from city

------------------------------but this can be done using merge statement--------
merge into city c
using (select 7 id,'lucknow' name) b
on(c.id=b.id)
when matched then
update set name=b.name
when not matched then
insert(id,name)values(b.id,b.name);

--select * from city

merge into city c
using (select 7 id,'lucknowUpdated' name) b
on(c.id=b.id)
when matched then
update set name=b.name
when not matched then
insert(id,name)values(b.id,b.name);


--select * from city

create table citydesination
(
id1 int,
name1 varchar(70)
)

select * from citydesination--no record
--lets insert one record 

insert into citydesination(id1,name1) values(7,'lucknowindestination')

select * from citydesination

--lets insert using merger

merge into citydesination d
using(select id,name from city)b
on (d.id1=b.id)
when matched then
update set name1=b.name
when not matched then
insert (id1,name1)values(b.id,b.name);

select * from citydesination

---------------------------How i used it in my stored procedure in my project-----------------

                     merge into #temp1 t

                     using (select empcode from tbl_SMM_Roster where dtdate >                                     dateadd(dd,-30,getdate())group by empcode)  smm

                     on t.userid=smm.empcode

                     when matched then

                     UPDATE SET [SMM DETAILS] = (select CONVERT(VARCHAR(11),dtDate,6) 

                     from tbl_SMM_Roster WITH(NOLOCK) where empcode=@userid

                     order by dtdate desc

                     OFFSET 0 ROWS

                     FETCH NEXT 1 ROWS ONLY) ;

                     merge into #temp1 t

                     using (select empcode from tbl_VW_Roster where dtdate >                                       dateadd(dd,-30,getdate())group by empcode)  vw

                     on t.userid=vw.empcode

                     when matched then

                     UPDATE SET [GTT DETAILS] = (select CONVERT(VARCHAR(11),dtDate,6) 

                     from tbl_VW_Roster WITH(NOLOCK) where empcode=@userid

                     order by dtdate desc

                     OFFSET 0 ROWS

                     FETCH NEXT 1 ROWS ONLY) ;

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