Translate the following question to SQL: When did customer with first name as Carole and last name as Bernhard became a customer?

Schema: CREATE TABLE Addresses (
    address_id INTEGER   PRIMARY KEY,
    line_1_number_building VARCHAR(80),
    city VARCHAR(50),
    zip_postcode VARCHAR(20),
    state_province_county VARCHAR(50),
    country VARCHAR(50)
);

CREATE TABLE Staff (
    staff_id INTEGER   PRIMARY KEY,
    staff_address_id INTEGER NOT NULL,
    nickname VARCHAR(80),
    first_name VARCHAR(80),
    middle_name VARCHAR(80),
    last_name VARCHAR(80),
    date_of_birth DATETIME,
    date_joined_staff DATETIME,
    date_left_staff DATETIME,
    FOREIGN KEY (staff_address_id) REFERENCES Addresses(address_id)
);

CREATE TABLE Vehicles (
    vehicle_id INTEGER   PRIMARY KEY,
    vehicle_details VARCHAR(255)
);

CREATE TABLE Customers (
    customer_id INTEGER   PRIMARY KEY,
    customer_address_id INTEGER NOT NULL,
    customer_status_code VARCHAR(15) NOT NULL,
    date_became_customer DATETIME,
    date_of_birth DATETIME,
    first_name VARCHAR(80),
    last_name VARCHAR(80),
    amount_outstanding DOUBLE,
    email_address VARCHAR(250),
    phone_number VARCHAR(255),
    cell_mobile_phone_number VARCHAR(255),
    FOREIGN KEY (customer_address_id) REFERENCES Addresses(address_id)
);

CREATE TABLE Customer_Payments (
    customer_id INTEGER NOT NULL  PRIMARY KEY,
    datetime_payment DATETIME NOT NULL  PRIMARY KEY,
    payment_method_code VARCHAR(10) NOT NULL,
    amount_payment DOUBLE,
    PRIMARY KEY (customer_id, datetime_payment),
    UNIQUE (customer_id, datetime_payment),
    FOREIGN KEY (customer_id) REFERENCES Customers(customer_id)
);

CREATE TABLE Lessons (
    lesson_id INTEGER   PRIMARY KEY,
    customer_id INTEGER NOT NULL,
    lesson_status_code VARCHAR(15) NOT NULL,
    staff_id INTEGER,
    vehicle_id INTEGER NOT NULL,
    lesson_date DATETIME,
    lesson_time VARCHAR(10),
    price DOUBLE,
    FOREIGN KEY (customer_id) REFERENCES Customers(customer_id),
    FOREIGN KEY (staff_id) REFERENCES Staff(staff_id),
    FOREIGN KEY (vehicle_id) REFERENCES Vehicles(vehicle_id)
);
user
SELECT date_became_customer FROM Customers WHERE first_name = "Carole" AND last_name = "Bernhard";
assistant