oracle merge if not exists code example

Example: oracle merge insert if not exists

MERGE INTO table1 t1
USING (SELECT 'value1' AS value1, value2
       FROM table2
       WHERE table2.type = 'ok') t2
ON (t1.value1 = t2.value1)
WHEN NOT MATCHED THEN
    INSERT VALUES (t2.value1, t2.value2);