Oracle: how to UPSERT (update or insert into a table?)

The UPSERT operation either updates or inserts a row in a table, depending if the table already has a row that matches the data:

if table t has a row exists that has key X:
    update t set mystuff... where mykey=X
else
    insert into t mystuff...

Since Oracle doesn't have a specific UPSERT statement, what's the best way to do this?

The MERGE statement merges data between two tables. Using DUAL allows us to use this command. Note that this is not protected against concurrent access.

create or replace
procedure ups(xa number)
as
begin
    merge into mergetest m using dual on (a = xa)
         when not matched then insert (a,b) values (xa,1)
             when matched then update set b = b+1;
end ups;
/
drop table mergetest;
create table mergetest(a number, b number);
call ups(10);
call ups(10);
call ups(20);
select * from mergetest;

A                      B
---------------------- ----------------------
10                     2
20                     1

The dual example above which is in PL/SQL was great becuase I wanted to do something similar, but I wanted it client side...so here is the SQL I used to send a similar statement direct from some C#

MERGE INTO Employee USING dual ON ( "id"=2097153 )
WHEN MATCHED THEN UPDATE SET "last"="smith" , "name"="john"
WHEN NOT MATCHED THEN INSERT ("id","last","name") 
    VALUES ( 2097153,"smith", "john" )

However from a C# perspective this provide to be slower than doing the update and seeing if the rows affected was 0 and doing the insert if it was.

An alternative to MERGE (the "old fashioned way"):

begin
   insert into t (mykey, mystuff) 
      values ('X', 123);
exception
   when dup_val_on_index then
      update t 
      set    mystuff = 123 
      where  mykey = 'X';
end;   

Another alternative without the exception check:

UPDATE tablename
    SET val1 = in_val1,
        val2 = in_val2
    WHERE val3 = in_val3;

IF ( sql%rowcount = 0 )
    THEN
    INSERT INTO tablename
        VALUES (in_val1, in_val2, in_val3);
END IF;

1. insert if not exists
2. update:

    
INSERT INTO mytable (id1, t1) 
  SELECT 11, 'x1' FROM DUAL 
  WHERE NOT EXISTS (SELECT id1 FROM mytble WHERE id1 = 11); 

UPDATE mytable SET t1 = 'x1' WHERE id1 = 11;