Question

Assume a table structure of MyTable(KEY, datafield1, datafield2...).

Often I want to either update an existing record, or insert a new record if it doesn’t exist.

Essentially:

IF (key exists)   run update command ELSE   run insert command 

What’s the best performing way to write this?

Answer 1

don’t forget about transactions. Performance is good, but simple (IF EXISTS..) approach is very dangerous.
When multiple threads will try to perform Insert-or-update you can easily get primary key violation.

Solutions provided by @Beau Crawford & @Esteban show general idea but error-prone.

To avoid deadlocks and PK violations you can use something like this:

begin tran if exists (select * from table with (updlock,serializable) where key = @key) begin    update table set ...    where key = @key end else begin    insert into table (key, ...)    values (@key, ...) end commit tran 

or

begin tran    update table with (serializable) set ...    where key = @key     if @@rowcount = 0    begin       insert into table (key, ...) values (@key,..)    end commit tran