Bookshelf: Primary Key-Foreign Key Relationships
Foreign key and a Primary key is used to define relationship between two tables in relational database. For example in Employee and. This column, or columns, is called the primary key (PK) of the table and In a foreign key reference, a link is created between two tables when. In the context of relational databases, a foreign key is a field (or collection of fields) in one table that uniquely identifies a row of another table or the same table. In simpler words, the foreign key is defined in a second table, but it refers to the primary key or In this case, the relationship between the two tables is called a one to many.
The reason it is a foreign key is that it is referring to a primary key, Person. BusinessEntityID, in the other table. I agree this is confusing, but it is allowed and not a bad practice.
Unlike primary keys, foreign keys can contain duplicate values.
Difference between Primary Key and Foreign Key
In the PersonPhone table can you find the other foreign key see answer at end of article? These are circled in green in the above diagram. These help to enforce referential integrity.
These constraints come into effect in several ways: They stop you from deleting a row from the primary key table.
Changes to primary key constraints are checked with foreign key constraints in related tables. Referential Integrity Although the main purpose of a foreign key constraint is to control the data that can be stored in the foreign key table, it also controls changes to data in the primary key table.
For example, if the row for a salesperson is deleted from the Sales. SalesPerson table, and the salesperson's ID is used for sales orders in the Sales.
Defining primary key–foreign key relationships
SalesOrderHeader table, the relational integrity between the two tables is broken; the deleted salesperson's sales orders are orphaned in the SalesOrderHeader table without a link to the data in the SalesPerson table.
A foreign key constraint prevents this situation.
The constraint enforces referential integrity by guaranteeing that changes cannot be made to data in the primary key table if those changes invalidate the link to data in the foreign key table. If an attempt is made to delete the row in a primary key table or to change a primary key value, the action will fail when the deleted or changed primary key value corresponds to a value in the foreign key constraint of another table.
To successfully change or delete a row in a foreign key constraint, you must first either delete the foreign key data in the foreign key table or change the foreign key data in the foreign key table, which links the foreign key to different primary key data.
Cascading Referential Integrity By using cascading referential integrity constraints, you can define the actions that the Database Engine takes when a user tries to delete or update a key to which existing foreign keys point. The following cascading actions can be defined.
For this constraint to execute, the foreign key columns must be nullable.
Foreign key - Wikipedia
For this constraint to execute, all foreign key columns must have default definitions. If a column is nullable, and there is no explicit default value set, NULL becomes the implicit default value of the column. If there are any AFTER triggers defined on the affected tables, these triggers fire after all cascading actions are performed.
These triggers fire in opposite order of the cascading action. If there are multiple triggers on a single table, they fire in random order, unless there is a dedicated first or last trigger for the table. However, one chain always fires all its triggers before another chain starts firing. Tables store huge chunks of data in them which usually extend to thousands of records all of which are unsorted and disorganized.
Fetching a particular data from those numerous records can be difficult at times or sometimes impossible. This is where Keys come to picture. Here, we will study about the two very important keys of the relational database schema and the difference between them: Primary key and Foreign Key. What is a Primary Key? A primary key is a special key which uniquely identifies each record in a table.
It is very important in the relational database to have a unique identifier in each row of a table and primary key is just the thing you need to uniquely identify a tuple within a table.
A tuple represents a set of value attributes in a relational database. A primary key may refer to a column or a set of columns in a relational database table used to implicitly identify all records in the table. The primary key must be unique for each record as it acts as a unique identifier and it should not contain Null values.
- Foreign key
- Primary and Foreign Key Constraints
- What is the Difference between a Primary Key and a Foreign Key?
Each database must have one and only one primary key.