SQL - Foreign Key
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Super key Now, given the definition above that a primary key can be made up of more than one column and must uniquely identify the rows, we could choose, for example, CustomerNo AND a column containing customer phone numbers as the primary key. To use the commonly-cited database, here is an excerpt from a Product table: The Northwind Database's Product Table Excerpt ProductID ProductName CategoryID QuantityPerU UnitPrice 1 Chai 1 10 boxes x 20 bags 18. Similarly, a row cannot be deleted as long as there is a reference to it from a referencing or child table.
NO ACTION The Database Engine raises an error and the delete or update action on the row in the parent table is rolled back. As shown in the following illustration, the ProductID and VendorID columns in the Purchasing. The values of the columns are not combined, they are compared using their data types.
SQL - Foreign Key - Some Basics of Relational Databases In a relational database, containing rows and columns, making it easy to search and manipulate. The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables.
In , a defines a relationship between two or more tables. That is, the data in one table is related to the data in the other. One table contains the primary key and the other table contains the foreign key. When we establish a relationship between the tables, we link the foreign key with the primary key. From that point on, any value in the foreign key field should match a value from the primary key field in the other table. Example of a one-to-many relationship. The CityId in the Customer table is the foreign key. In the above example, the CityId field in the Customer table is a foreign key. It is linked to the CityId field in the City table which is a primary key. For any given record, the value of Customer. CityId will match the value of City. So foreign keys are an essential part of relational database systems. This is because the value of the primary field is a unique identifier. Therefore, the value of the foreign key field is the unique identifier for the record in the other table. So we know that the foreign key value refers to only that single.
This process may be recursively repeated for subsequent UPDATE or DELETE operations. This email address is already registered. In this case, the relationship between the two tables is called a between the referenced table and the referencing table. Alternate key A Candidate key that is not the primary key is called an Alternate key. This is done by declaring a constraint after all of your columns have been declared. The rest of them as they are alternates only. The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables. It is possible to use one or more columns as the primary key; however, how do you -- and how many -- to choose?.
