Translate the following question to SQL: What are the theme and year for all exhibitions that have a ticket price under 15?

Schema: CREATE TABLE artist (
    Artist_ID INT   PRIMARY KEY,
    Name TEXT,
    Country TEXT,
    Year_Join INT,
    Age INT,
    UNIQUE (Artist_ID)
);

CREATE TABLE exhibition (
    Exhibition_ID INT   PRIMARY KEY,
    Year INT,
    Theme TEXT,
    Artist_ID INT,
    Ticket_Price REAL,
    UNIQUE (Exhibition_ID),
    FOREIGN KEY (Artist_ID) REFERENCES artist(Artist_ID)
);

CREATE TABLE exhibition_record (
    Exhibition_ID INT   PRIMARY KEY,
    Date TEXT   PRIMARY KEY,
    Attendance INT,
    PRIMARY KEY (Exhibition_ID, Date),
    UNIQUE (Exhibition_ID, Date),
    FOREIGN KEY (Exhibition_ID) REFERENCES exhibition(Exhibition_ID)
);
user
SELECT theme ,  YEAR FROM exhibition WHERE ticket_price  <  15
assistant