Update table but skipping some rows with specific condition - sql-server

Question

I have a table called body_scan that looks like this:
body_no tag
--------------------
1 noscan
2 noscan
3 missing
4 noscan
5 missing
I also have a list that I can load into a temp table like so
tag_no
------
aaa
bbb
ccc
What I need to be able to do is to update the body_scan table with the tag numbers in the temporary table.
You will notice that there are only 3 tags in the temp table but 5 in the body_scan table. I need to update the tag value "noscan" with values from the temp table and leave the missing as they are..
The order of the tags in the temporary table is the same as the order of body_no from the body_scan table.
So yes, I did consider the row_number() function. But I'm just not 100% sure how to define the join correctly..
How do I achieve this please?
The desired result is :
body_no tag
-------------------
1 aaa
2 bbb
3 missing
4 ccc
5 missing

Answer 1

Firstly, you need to preserve the input file order of data by adding an identity field to the temp_table (note that some ETL tools insert data in parallel and that messes things up so you might even need to add this column to the file)
Once you've done that, you need to generate a key in body_scan that you can join to. This is simply ROW_NUMBER() over the existing table, excluding the missing rows
This returns the row and what it should be matched to in temp_table
SELECT
body_no,
ROW_NUMBER() OVER (ORDER BY body_no) RN
FROM body_scan
WHERE tag<> 'missing';
This joins in the temp table (assumes your ordinal column is called RowID)
SELECT T1.body_no, T1.tag, T1.RN, T2.tag_no
FROM
(
SELECT
body_no,tag,
ROW_NUMBER() OVER (ORDER BY body_no) RN
FROM body_scan
WHERE tag<> 'missing'
) T1
INNER JOIN
temp_table T2
ON T1.RN=T2.RowID;
This updates it back to the table:
UPDATE TGT
SET tag=SRC.tag_no
FROM body_scan TGT
INNER JOIN
(
SELECT T1.body_no, T2.tag_no
FROM
(
SELECT
body_no,tag,
ROW_NUMBER() OVER (ORDER BY body_no) RN
FROM body_scan
WHERE tag<> 'missing'
) T1
INNER JOIN
temp_table T2
ON T1.RN=T2.RowID
) SRC
ON SRC.body_no=TGT.body_no;
(There's half a dozen ways to write that final statement but I prefer this way as you can see the dataset you're updating from in the subselect)

Answer 2

I cant understand your explanation and command discussion. I workout(in SQL 2012) to achieve your OUTPUT table. As,
update a
set a.tag = t.tag
from (
select m.*, ROW_NUMBER() over(partition by m.tag order by m.rn)trn from(
select *, row_number() over(partition by (select null) order by (select null)) rn from body_scan --set order what the order of actual table's order
) m --set row number for noscan rows
) a
join(
select *, ROW_NUMBER() over(order by (select null)) rn from #temp --set order what the order of actual table order
) t
on a.trn = t.rn and a.tag <> 'missing' -- join to noscan rows using row numbers
OUTPUT:
body_no tag
--------------
1 aaa
2 bbb
3 missing
4 ccc
5 missing