Translate the following question to SQL: What is the largest and smallest customer codes?

Schema: CREATE TABLE Addresses (
    address_id INTEGER   PRIMARY KEY,
    address_details VARCHAR(255)
);

CREATE TABLE Staff (
    staff_id INTEGER   PRIMARY KEY,
    staff_gender VARCHAR(1),
    staff_name VARCHAR(80)
);

CREATE TABLE Suppliers (
    supplier_id INTEGER   PRIMARY KEY,
    supplier_name VARCHAR(80),
    supplier_phone VARCHAR(80)
);

CREATE TABLE Department_Store_Chain (
    dept_store_chain_id INTEGER   PRIMARY KEY,
    dept_store_chain_name VARCHAR(80)
);

CREATE TABLE Customers (
    customer_id INTEGER   PRIMARY KEY,
    payment_method_code VARCHAR(10) NOT NULL,
    customer_code VARCHAR(20),
    customer_name VARCHAR(80),
    customer_address VARCHAR(255),
    customer_phone VARCHAR(80),
    customer_email VARCHAR(80)
);

CREATE TABLE Products (
    product_id INTEGER   PRIMARY KEY,
    product_type_code VARCHAR(10) NOT NULL,
    product_name VARCHAR(80),
    product_price DECIMAL(19,4)
);

CREATE TABLE Supplier_Addresses (
    supplier_id INTEGER NOT NULL  PRIMARY KEY,
    address_id INTEGER NOT NULL  PRIMARY KEY,
    date_from DATETIME NOT NULL,
    date_to DATETIME,
    PRIMARY KEY (supplier_id, address_id),
    UNIQUE (supplier_id, address_id),
    FOREIGN KEY (supplier_id) REFERENCES Suppliers(supplier_id),
    FOREIGN KEY (address_id) REFERENCES Addresses(address_id)
);

CREATE TABLE Customer_Addresses (
    customer_id INTEGER NOT NULL  PRIMARY KEY,
    address_id INTEGER NOT NULL  PRIMARY KEY,
    date_from DATETIME NOT NULL,
    date_to DATETIME,
    PRIMARY KEY (customer_id, address_id),
    UNIQUE (customer_id, address_id),
    FOREIGN KEY (customer_id) REFERENCES Customers(customer_id),
    FOREIGN KEY (address_id) REFERENCES Addresses(address_id)
);

CREATE TABLE Customer_Orders (
    order_id INTEGER   PRIMARY KEY,
    customer_id INTEGER NOT NULL,
    order_status_code VARCHAR(10) NOT NULL,
    order_date DATETIME NOT NULL,
    FOREIGN KEY (customer_id) REFERENCES Customers(customer_id)
);

CREATE TABLE Department_Stores (
    dept_store_id INTEGER   PRIMARY KEY,
    dept_store_chain_id INTEGER,
    store_name VARCHAR(80),
    store_address VARCHAR(255),
    store_phone VARCHAR(80),
    store_email VARCHAR(80),
    FOREIGN KEY (dept_store_chain_id) REFERENCES Department_Store_Chain(dept_store_chain_id)
);

CREATE TABLE Departments (
    department_id INTEGER   PRIMARY KEY,
    dept_store_id INTEGER NOT NULL,
    department_name VARCHAR(80),
    FOREIGN KEY (dept_store_id) REFERENCES Department_Stores(dept_store_id)
);

CREATE TABLE Order_Items (
    order_item_id INTEGER   PRIMARY KEY,
    order_id INTEGER NOT NULL,
    product_id INTEGER NOT NULL,
    FOREIGN KEY (product_id) REFERENCES Products(product_id),
    FOREIGN KEY (order_id) REFERENCES Customer_Orders(order_id)
);

CREATE TABLE Product_Suppliers (
    product_id INTEGER NOT NULL  PRIMARY KEY,
    supplier_id INTEGER NOT NULL  PRIMARY KEY,
    date_supplied_from DATETIME NOT NULL,
    date_supplied_to DATETIME,
    total_amount_purchased VARCHAR(80),
    total_value_purchased DECIMAL(19,4),
    PRIMARY KEY (product_id, supplier_id),
    UNIQUE (product_id, supplier_id),
    FOREIGN KEY (product_id) REFERENCES Products(product_id),
    FOREIGN KEY (supplier_id) REFERENCES Suppliers(supplier_id)
);

CREATE TABLE Staff_Department_Assignments (
    staff_id INTEGER NOT NULL  PRIMARY KEY,
    department_id INTEGER NOT NULL  PRIMARY KEY,
    date_assigned_from DATETIME NOT NULL,
    job_title_code VARCHAR(10) NOT NULL,
    date_assigned_to DATETIME,
    PRIMARY KEY (staff_id, department_id),
    UNIQUE (staff_id, department_id),
    FOREIGN KEY (staff_id) REFERENCES Staff(staff_id),
    FOREIGN KEY (department_id) REFERENCES Departments(department_id)
);
user
SELECT max(customer_code) ,  min(customer_code) FROM Customers
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