Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE Order_items ( product_id VARCHAR, order_id VARCHAR )

### Question ###
Show the product ids and the number of unique orders containing each product.

### Accurate SQL ###

SELECT product_id, COUNT(DISTINCT order_id) FROM Order_items GROUP BY product_id