--Creating the tables and insert some test data
create table tbl (i int not null primary key, c char(5))
go

insert into tbl (i,c)
select 1, 'abc'
union 
select 2, 'def'
union 
select 3, 'ghi'
go

create table StagingTbl (i int not null primary key, c char(5))
go


insert into StagingTbl (i,c)
select 3, 'ghijk'
union 
select 4, 'lmnop'
go

--First step is to update column c according to i
--I do it with an update statement that uses
--from clause.  You can read about it in BOL
update tbl
set tbl.c = StagingTbl.c
from tbl inner join StagingTbl on tbl.i = StagingTbl.i

--The insert is done with select that is doing a left
--join and inserts only records that were not found
--in tbl1.
insert into tbl (i,c)
select StagingTbl.i, StagingTbl.c
from StagingTbl left join tbl on StagingTbl.i = tbl.i
where tbl.i is null
go

--Check the results
select * from Tbl

--cleanup
drop table tbl
go
drop table StagingTbl

--DROP TABLE #Destination, #Staging

CREATE TABLE #Destination (a INT IDENTITY(1,1) PRIMARY KEY, Data VARCHAR(36) NULL)
CREATE TABLE #Staging (a INT IDENTITY(5,1) PRIMARY KEY, Data VARCHAR(36) NULL)

-- Test data
SET NOCOUNT ON
GO
INSERT #Destination (Data) SELECT '';
INSERT #Staging (Data) SELECT CONVERT(VARCHAR(36), NEWID());
GO 10

-- Show the 'before'
SELECT * FROM #Destination; SELECT * FROM #Staging

DECLARE	@KeysInserted TABLE (a INT PRIMARY KEY)

SET IDENTITY_INSERT #Destination ON

-- Insert new rows, remembering the keys
INSERT	#Destination (a, Data)
OUTPUT	inserted.a INTO @KeysInserted
SELECT	S.a, S.Data
FROM	#Staging AS S
WHERE	NOT EXISTS (SELECT 1 FROM #Destination AS D WHERE S.a = D.a)

SET IDENTITY_INSERT #Destination OFF

-- Update
UPDATE	D
SET		Data = S.Data
FROM	#Staging AS S
JOIN	#Destination AS D ON D.a = S.a
WHERE	NOT EXISTS (SELECT 1 FROM @KeysInserted AS KI WHERE KI.a = S.a)

-- The 'after'
SELECT	*
FROM	#Destination