What is the real difference between one to many and many to one relationship?
In fact the Customer might also have a one-to-many relationship with ShippingAddress and SalesCall tables and yet have no additional columns added to the Customer table. The Customer has no extra columns which describe the relationship with Order. What is important to realize is that typically the one-to-many relationship doesn't actually add any columns to the table that is the "one". Jim Kenshaw (customer-id #2) has only 1 order for $158.01. In the above data rows, if we look at the customer_id id column, we see that Bill Smith (customer-id #1) has 2 orders associated with him: one for $12.34 and one for $7.58. Then for a Order to be associated with a Customer, many SQL implementations add to the Order table a column which stores the id of the associated Customer (in this schema customer_id: id,date,amount,customer_id For example, if the customer has columns id and name: id,name In addition, the schema which supports the relationship may be represented differently in the Customer and Order tables. This conceptual difference is important for mental representation. In our example, many Orders may be associated to one Customer. In the opposite many-to-one relationship, the local table may have many rows that are associated with one row in another table. In the example from SQL for beginners, one Customer may be associated to many Orders. In a one-to-many relationship, the local table has one row that may be associated with many rows in another table. Mostly the difference is one of perspective though. There are conceptual differences between these terms that should help you visualize the data and also possible differences in the generated schema that should be fully understood. What is the real difference between one-to-many and many-to-one relationship?