can someone help me to explain on this why is the parent_product_type_code is referencint to it’s own table ? what would be the values to be inputted ?, I am confuse because it reference to itself .
product_types
+----+--------------------------+
| PK | product_type_code |
+----+--------------------------+
| FK | parent_product_type_code |
| | product_type_description |
+----+--------------------------+
products
+----+---------------------+
| PK | product_id |
+----+---------------------+
| FK | product_type_code |
| | product_name |
| | unit_price |
| | product_description |
+----+---------------------+