### Database 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,
`address_id` INTEGER NOT NULL,
`date_from` DATETIME NOT NULL,
`date_to` DATETIME,
PRIMARY KEY (`supplier_id`, `address_id`),
FOREIGN KEY (`address_id` ) REFERENCES `Addresses`(`address_id` ),
FOREIGN KEY (`supplier_id` ) REFERENCES `Suppliers`(`supplier_id` )
)

CREATE TABLE `Customer_Addresses` (
`customer_id` INTEGER NOT NULL,
`address_id` INTEGER NOT NULL,
`date_from` DATETIME NOT NULL,
`date_to` DATETIME,
PRIMARY KEY (`customer_id`, `address_id`),
FOREIGN KEY (`address_id` ) REFERENCES `Addresses`(`address_id` ),
FOREIGN KEY (`customer_id` ) REFERENCES `Customers`(`customer_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 (`order_id` ) REFERENCES `Customer_Orders`(`order_id` ),
FOREIGN KEY (`product_id` ) REFERENCES `Products`(`product_id` )
)

CREATE TABLE `Product_Suppliers` (
`product_id` INTEGER NOT NULL,
`supplier_id` INTEGER NOT NULL,
`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`),
FOREIGN KEY (`supplier_id` ) REFERENCES `Suppliers`(`supplier_id` ),
FOREIGN KEY (`product_id` ) REFERENCES `Products`(`product_id` )
)

CREATE TABLE `Staff_Department_Assignments` (
`staff_id` INTEGER NOT NULL,
`department_id` INTEGER NOT NULL,
`date_assigned_from` DATETIME NOT NULL,
`job_title_code` VARCHAR(10) NOT NULL,
`date_assigned_to` DATETIME,
PRIMARY KEY (`staff_id`, `department_id`),
FOREIGN KEY (`department_id` ) REFERENCES `Departments`(`department_id` ),
FOREIGN KEY (`staff_id` ) REFERENCES `Staff`(`staff_id` )
)

### Question
Give the product id for the product that was ordered most frequently.

### SQL
SELECT product_id FROM order_items GROUP BY product_id ORDER BY count(*) DESC LIMIT 1