I am afraid there is no sql92 answer to this question.
For sq192 an approach would be to insert every row and use grouping functions for reporting.

MySQL has a special construct for this. Assume the 'username' column below is UNIQUE:

INSERT INTO users (username, email) VALUES ('Jo', 'jo@email.com')
ON DUPLICATE KEY UPDATE email = 'jo@email.com'

The 'ON DUPLICATE KEY' statement only works on PRIMARY KEY and UNIQUE columns.

How about this:
IF (EXISTS (SELECT * FROM AA_TestTable AS t1
  WHERE t1.ord_num = 'FFF'))
begin
  UPDATE AA_TestTable
  SET ord_qty = 999
  WHERE ord_num = 'FFF'
end
else
begin
  INSERT INTO AA_TestTable (ord_num, top_assy, ord_qty)
  VALUES('GGG', 'XYZ', 567)
end 

INSERT INTO users (username)
SELECT 'Jo'
WHERE 'Jo'
  NOT IN (SELECT username FROM users)

UPDATE users
SET email = 'jo@email.com'
WHERE username = 'Jo'

I use this method a lot. Obviously, 'Jo' would usually either be a variable or a field selected from another table.

This opption worked wonders! Thanks for posting

(I assume there is a typo in the last statement, where ord_num should be set to 'FFF').

You should be VERY CAREFUL with things like this. If you can't afford to set the transaction isolation level to SERIALIZABLE, some other transaction could add the row with ord_num='FFF' after you've tested for its existence, but before you've inserted it. This way you end up with violated primary constraint and error in one of these transactions.

This is a variation that works for tables with multiple primary keys.
If you have a users table with columns of username, dept, and age, and primary keys of username and dept, then this will only insert a user if it doesn't exist already.

INSERT INTO users (username, dept, age) SELECT username='mp', dept='tax', age=5 WHERE (SELECT COUNT (*) FROM users WHERE username='mp' AND dept='tax')=0;

I initially found this syntax confusing, but here's how I 'parsed' it.
This part returns 0 if it doesn't exist:
SELECT COUNT (*) FROM users WHERE username='mp' AND dept='tax'
This part creates a record-like row with static values and represents the record to insert:
SELECT username='mp', dept='tax', age=5
The WHERE-clause following the above part will only return the value to insert if the count is 0.