The statement is called MERGE. Look it up, I m too lazy.

Beware, though, that MERGE is not atomic, which could cause the following effect (thanks, Marius):

SESS1:

create table t1 (pk int primary key, i int);
create table t11 (pk int primary key, i int);
insert into t1 values(1, 1);
insert into t11 values(2, 21);
insert into t11 values(3, 31);
commit;

SESS2: insert into t1 values(2, 2);

SESS1:

MERGE INTO t1 d
USING t11 s ON (d.pk = s.pk)
WHEN NOT MATCHED THEN INSERT (d.pk, d.i) VALUES (s.pk, s.i);

SESS2: commit;

SESS1: ORA-00001

Coming late to the party, but...

With oracle 11.2.0.1 there is a semantic hint that can do this: IGNORE_ROW_ON_DUPKEY_INDEX

Example:

insert /*+ IGNORE_ROW_ON_DUPKEY_INDEX(customer_orders,pk_customer_orders) */
  into customer_orders
       (order_id, customer, product)
values (    1234,     9876,   K598 )
     ;

UPDATE: Although this hint works (if you spell it correctly), there are better approaches which don t require Oracle 11R2:

First approach—direct translation of above semantic hint:

begin
  insert into customer_orders
         (order_id, customer, product)
  values (    1234,     9876,   K698 )
  ;
  commit;
exception
  when DUP_VAL_ON_INDEX
  then ROLLBACK;
end;

Second aproach—a lot faster than both above hints when there s a lot of contention:

begin
    select count (*)
    into   l_is_matching_row
    from   customer_orders
    where  order_id = 1234
    ;

    if (l_is_matching_row = 0)
    then
      insert into customer_orders
             (order_id, customer, product)
      values (    1234,     9876,   K698 )
      ;
      commit;
    end if;
exception
  when DUP_VAL_ON_INDEX
  then ROLLBACK;
end;

We can combine the DUAL and NOT EXISTS to achieve your requirement:

INSERT INTO schema.myFoo ( 
    primary_key, value1, value2
) 
SELECT
     bar ,  baz ,  bat  
FROM DUAL
WHERE NOT EXISTS (
    SELECT 1 
    FROM schema.myFoo
    WHERE primary_key =  bar 
);

This only inserts if the item to be inserted is not already present.

Works the same as:

if not exists (...) insert ... 

in T-SQL

insert into destination (DESTINATIONABBREV) 
  select  xyz  from dual 
  left outer join destination d on d.destinationabbrev =  xyz  
  where d.destinationid is null;

may not be pretty, but it s handy :)

If you do NOT want to merge in from an other table, but rather insert new data... I came up with this. Is there perhaps a better way to do this?

MERGE INTO TABLE1 a
    USING DUAL
    ON (a.C1_pk= 6)
WHEN NOT MATCHED THEN
    INSERT(C1_pk, C2,C3,C4)
    VALUES (6, 1,0,1);

It that code is on the client then you have many trips to the server so to eliminate that.

Insert all the data into a temportary table say T with the same structure as myFoo

Then

insert myFoo
  select *
     from t
       where t.primary_key not in ( select primary_key from myFoo) 

This should work on other databases as well - I have done this on Sybase

It is not the best if very few of the new data is to be inserted as you have copied all the data over the wire.

DECLARE
   tmp NUMBER(3,1);
BEGIN
  SELECT COUNT(content_id) INTO tmp FROM contents WHERE (condition);
  if tmp != 0 then
    INSERT INTO contents VALUES (...);
  else
    INSERT INTO contents VALUES (...);
  end if;
END;

I used the code above. It is long, but, simple and worked for me. Similar, to Micheal s code.

If your table is "independent" from others (I mean, it will not trigger a cascade delete or will not set any foreign keys relations to null), a nice trick could be to first DELETE the row and then INSERT it again. It could go like this:

DELETE FROM MyTable WHERE prop1 = aaa ; //assuming it will select at most one row!

INSERT INTO MyTable (prop1, ...) VALUES ( aaa , ...);

If your are deleting something which does not exist, nothing will happen.

This is an answer to the comment posted by erikkallen:

You don t need a temp table. If you only have a few rows, (SELECT 1 FROM dual UNION SELECT 2 FROM dual) will do. Why would your example give ORA-0001? Wouldn t merge take the update lock on the index key and not continue until Sess1 has either committed or rolled back? – erikkallen

Well, try it yourself and tell me whether you get the same error or not:

SESS1:

create table t1 (pk int primary key, i int);
create table t11 (pk int primary key, i int);
insert into t1 values(1, 1);
insert into t11 values(2, 21);
insert into t11 values(3, 31);
commit;

SESS2: insert into t1 values(2, 2);

SESS1:

MERGE INTO t1 d
USING t11 s ON (d.pk = s.pk)
WHEN NOT MATCHED THEN INSERT (d.pk, d.i) VALUES (s.pk, s.i);

SESS2: commit;

SESS1: ORA-00001

INSERT INTO schema.myFoo ( primary_key        , value1          , value2         )
                         SELECT  bar1  AS primary_key , baz1  AS value1 , bat1  AS value2 FROM DUAL WHERE (SELECT 1 AS value FROM schema.myFoo WHERE LOWER(primary_key) = bar1  AND ROWNUM=1) is null;