How can I iterate through SQL results like a for loop and an array?

Question

I have a table, and I want to select only the single column of row IDs from it, but in a specific order. Then, I want to loop through that column like below:

for (i=0; i<rows.length; i++)
{
  if(i==rows.length-1)
    UPDATE myTable SET nextID = NULL WHERE ID = rows[i]
  ELSE
    UPDATE myTable SET nextID = rows[i+1] WHERE ID = rows[i]
}

I just dont know how to access the results of my select statement with an index like that. Is there a way of doing this in sql server?

Answer 1

You could use a cursor, or you could use something a bit smarter.

Your example should be able to be written fairly easily along the lines of:

update mytable set nextID = LEAD(id,1) over (order by id)

Lead(id,1) will grab the next id, 1 row ahead in the record set and update the nextID field with it. If it can't find one it will return null. No looping or conditional logic needed!

edit: I forgot the over clause. This is the part that tells it how you would like it ordered for the lead

Answer 2

Since you didn't provide many details, let's pretend your table looks something like this:

create table MyTable (
  Id int not null primary key,
  Name varchar(50) not null,
  NextId int
)

I want to select only the single column of row IDs from it, but in a specific order

Let's just say that in this case, you decide to order the rows alphabetically by Name . So let's pretend that the select statement that you want to loop through looks like this:

select Id
  from MyTable
 order by Name

That being the case, instead of looping through the rows and attempting to update each row using the pseudo-code you provided, you can replace the whole thing with a single update statement that will perform the exact same work:

with cte as (
  select *,
         NewNextId = lead(Id) over (order by Name)
    from MyTable
)
update cte
   set NextId = NewNextId

Just make sure to adjust the order by clause to whatever your specific order really is. I just used Name in my example, but it might be something else in your case.