SQL update order in relation ship based on ID - sql

Question

I wonder how to update order in this table for many-to-many relationship using SQL based on PostsId.
So my table now looks like:
I'm using SQL Server
BlogsId
PostsId
Order
1
1
1
0
2
2
1
0
3
3
2
0
3
4
2
0
3
5
3
0
3
6
3
0
but I want to update Order using SQL to this:
BlogsId
PostsId
Order
1
1
1
1
2
2
1
2
3
3
2
1
3
4
2
2
3
5
3
1
3
6
3
2
So for example: Blog with Id 3 is the first blog in Post with Id 2, Blog with Id 4 is the second Blog in Post with Id 2 and etc...
I've tried this:
DECLARE #myVar int
SET #myVar = 0
UPDATE [dbo].[BlogPost]
SET #myVar = [Order] = #myVar + 1
but then I got this:
BlogsId
PostsId
Order
1
1
1
1
2
2
1
2
3
3
2
3
3
4
2
4
3
5
3
5
3
6
3
6
So, I think I should do something in WHERE part (with Distinct maybe) but I don't know exactly what. I could write something in C# to do what I want but I don't know how to write something like this in SQL.

Answer 1

Physically maintaining an order or sequence of rows is rarely a good idea and can lead to data inconsistencies and other unforseen issues.
You would be better off creating a view that provides the additional Order column which you can do using row_number()
Create view BlogPosts as
select *,
Row_Number() over(partition by PostsId order by BlogsId) as [Order]
from blogpost;
If you really want to update an actual column in the table you could use a CTE
with b as (
select *,
Row_Number() over(partition by PostsId order by BlogsId) as seq
from blogpost
)
update b
set [Order] = seq;

Answer 2

You can update from a calculated row_number.
update t
set [Order] = rn
from (
select BlogsId, PostsId, [Order]
, rn = row_number() over (partition by PostsId order by BlogsId asc)
from BlogPost
) t
where ([Order] is null or [Order]!=rn);
select *
from BlogPost
order by BlogsId, PostsId
BlogsId
PostsId
Order
1
1
1
2
1
2
3
2
1
4
2
2
5
3
1
6
3
2
Demo on db<>fiddle here