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Now you can perceive (look at) the relation the other way around, Bar::Foo is many-to-one Therefore the relation Foo::Bar is one-to-many Since Bar.Foo is a Reference, a Foreign Key, and there is no unique index on it, there can be many rows for any given value of Foo Since Foo.Foo is a Primary Key, it is unique, there is only one row for any given value of Foo REFERENCES Foo(Foo) - pk in referenced table Not the other way around CREATE TABLE Foo (įoo CHAR(10) NOT NULL, - foreign key to FooĬONSTRAINT Foo_HasMany_Bars - constraint nameįOREIGN KEY (Foo) - fk in (this) referencing table (Your front end may do helpful or confusing things, but that is a different story.)Ī Foreign Key in one table (the referenc ing table)Ī Primary Key in another table (the referenc ed table) In SQL, there is only one kind of relationship, it is called a Reference.
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Which mechanism depends on your schema and SQL implementation. In this case, the one-to-many and many-to-one is all conceptual since there are no schema changes between them. Instead of adding a customer_id field to the Order table, there could be Customer_Order table that contains keys for both the Customer and Order. However, it is also possible for there to be another table that describes the Customer and Order relationship, so that no additional fields need to be added to the Order table. To associated order #10 for $12.34 to Bill Smith, we assign the customer_id column to Bill Smith's id 1. However, for a many-to-one relationship to be described, often an id column is added to the "many" table which is a foreign-key to the "one" table - in this case a customer_id column is added to the Order. 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
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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?