The following example fails because the INSERT specification of the WHEN NOT MATCHED clause specifies a column, z1, from the target table t1, which is an illegal operation.
MERGE INTO t1 USING (SELECT x2,y2, z3 FROM t2, t3 WHERE y2=10) AS s ON x1=y2 AND t4.x4=z2 WHEN MATCHED THEN UPDATE SET z1=10 WHEN NOT MATCHED THEN INSERT (x1,y1, z1) VALUES (y2, t1.z1, x2);