Translate the following question to SQL: Find the person who has exactly one friend.

Schema: CREATE TABLE Person (
    name varchar(20)   PRIMARY KEY,
    age INTEGER,
    city TEXT,
    gender TEXT,
    job TEXT,
    UNIQUE (name)
);

CREATE TABLE PersonFriend (
    name varchar(20),
    friend varchar(20),
    year INTEGER,
    FOREIGN KEY (friend) REFERENCES Person(name),
    FOREIGN KEY (name) REFERENCES Person(name)
);
user
SELECT name FROM PersonFriend GROUP BY name HAVING count(*)  =  1
assistant