TSQL procedural issue and advice - sql-server

Question

I am looking for advice on the best way to accomplish the following
I have a table in SQL Server that holds downloaded data from an external system. I need to use it to update another database. Some records will be inserts and others will be updates. There is a comment table and a main table to insert/update. Comments are linked by an ID created in the comments table and stored in a column of the main table record. (one to one relationship)
So insert into comment table and get a scope_identity return value and then use that as part of the insert statement for the main table.
The updates get the comment ID from the record in the main table and then update the comment table where necessary and also the main table where necessary
EG Table has 5 records
Get first record
If exists in database
get commentID column from comment table and update comment and main table
If not exists
insert into comment table and return comment ID and insert the record into the main table with that comment ID
get the next record
I'm struggling to figure out how to best do this in SQL Server. Can't find the right combination of cursor, while loops, stored procedure etc. Haven't done much by way of procedural work in SQL Server.
Any advice/help is greatly appreciated
Thanks
Habo. I appreciate the feedback. I do struggle to write a clear concise question. The linked page provides good advice. Hope this script below helps clarify.
Thanks again.
USE TEMPDB
--TABLE TO HOLD JOB RECORDS
create table tbl_jobs(
jobnumber varchar(16) primary key clustered,
jobdesc varchar(50),
commentID int
)
GO
INSERT INTO tbl_jobs VALUES ('Job1','Desc1', '1')
INSERT INTO tbl_jobs VALUES ('Job2','Desc2', '2')
INSERT INTO tbl_jobs VALUES ('Job3','Desc3', '3')
--TABLE TO HOLD JOB RECORD COMMENTS
create table tbl_jobComments(
commentID INT IDENTITY(1,1) NOT NULL,
comment text
)
GO
Insert into tbl_jobComments VALUES ('Comment1')
Insert into tbl_jobComments VALUES ('Comment2')
Insert into tbl_jobComments VALUES ('Comment3')
--TABLE TO HOLD RECORDS DOWNLOADED FROM EXTERNAL SYSTEM
create table tbl_updates(
jobnumber varchar(16) primary key clustered,
jobdesc varchar(50),
comment text
)
GO
INSERT INTO tbl_updates VALUES ('Job1','Desc1Modified', 'Comment1')
INSERT INTO tbl_updates VALUES ('Job2','Desc2', 'Comment2')
INSERT INTO tbl_updates VALUES ('Job3','Desc3Modified', 'Comment3')
INSERT INTO tbl_updates VALUES ('Job4','Desc4', 'Comment4')
GO
--OUTPUT FROM tbl_Jobs
+-----------+---------+-----------+
| jobnumber | jobdesc | commentID |
+-----------+---------+-----------+
| Job1 | Desc1 | 1 |
| Job2 | Desc2 | 2 |
| Job3 | Desc3 | 3 |
+-----------+---------+-----------+
--OUTPUT FROM tbl_JobComments
+-----------+----------+
| commentID | comment |
+-----------+----------+
| 1 | Comment1 |
| 2 | Comment2 |
| 3 | Comment3 |
+-----------+----------+
--OUTPUT FROM tbl_updates
+-----------+---------------+-----------+
| jobnumber | jobdesc | comment |
+-----------+---------------+-----------+
| Job1 | Desc1Modified | Comment1 |
| Job2 | Desc2 | Comment2a |
| Job3 | Desc3Modified | Comment3 |
| Job4 | Desc4 | Comment4 |
+-----------+---------------+-----------+
--DESIRED RESULTS tbl_jobs
+-----------+-----------------+-----------+
| jobnumber | jobdesc | commentID |
+-----------+-----------------+-----------+
| Job1 | Desc1Modified | 1 |
| Job2 | Desc2 | 2 |
| Job3 | Desc3Modified | 3 |
| Job4 | Desc4 | 4 |
+-----------+---------+-------------------+
--DESIRED RESULTS tbl_jobs_comments
+-----------+-----------+
| commentID | comment |
+-----------+-----------+
| 1 | Comment1 |
| 2 | Comment2a |
| 3 | Comment3 |
| 4 | Comment4 |
+-----------+-----------+

Answer 1

You can break this into 2 statements, an update and an insert query
(This assumes there is only 1 comment per ID)
UPDATE maintable
SET Comment=upd.comment
FROM maintable mt
JOIN updatestable upd
ON mt.id=upd.id
then insert what is missing:
INSERT INTO maintable (id,comment)
SELECT id, comment
FROM updatestable
WHERE id NOT IN (SELECT id FROM maintable)