Question

I have a question regarding performance of SQL Server.

Suppose I have a table persons with the following columns: id, name, surname.

Now, I want to insert a new row in this table. The rule is the following:

1. If id is not present in the table, then insert the row.

2. If id is present, then update.

I have two solutions here:

First:

update persons
  set id=@p_id, name=@p_name, surname=@p_surname
where id=@p_id
if @@ROWCOUNT = 0 
  insert into persons(id, name, surname)
  values (@p_id, @p_name, @p_surname)

Second:

if exists (select id from persons where id = @p_id)
  update persons
    set id=@p_id, name=@p_name, surname=@p_surname
  where id=@p_id
else
  insert into persons(id, name, surname)
  values (@p_id, @p_name, @p_surname)

What is a better approach? It seems like in the second choice, to update a row, it has to be searched two times, whereas in the first option – just once. Are there any other solutions to the problem? I am using MS SQL 2000.

Answer 1

Both work fine, but I usually use option 2 (pre-mssql 2008) since it reads a bit more clearly. I wouldn’t stress about the performance here either…If it becomes an issue, you can use NOLOCK in the exists clause. Though before you start using NOLOCK everywhere, make sure you’ve covered all your bases (indexes and big picture architecture stuff). If you know you will be updating every item more than once, then it might pay to consider option 1.

Option 3 is to not use destructive updates. It takes more work, but basically you insert a new row every time the data changes (never update or delete from the table) and have a view that selects all the most recent rows. It’s useful if you want the table to contain a history of all its previous states, but it can also be overkill.