Show names of actors that have appeared in musical with name "The Phantom of the Opera".
SELECT T1.Name FROM actor AS T1 JOIN musical AS T2 ON T1.Musical_ID  =  T2.Musical_ID WHERE T2.Name  =  "The Phantom of the Opera"
actors that have appeared in refers to JOIN actor ON musical; musical with name 'The Phantom of the Opera' refers to WHERE musical.Name = 'The Phantom of the Opera';