Primary key and foreign key

What is the difference between a primary and a foreign key? Can a foreign key act as a primary key? How do I create a foreign key?

There isn’t any special code, configurations, or table definitions you need to place to officially “designate” a foreign key. In the diagram below look at the SalesOrderHeader table. Foreign key is a field in the table that is primary key in another table. These are important database objects.

This topic contains the following sections. Primary Key of another table. A primary key segment can’t have NULL qualities. The table containing the foreign key is called the child table, and the table containing the candidate key is called the referenced or parent table. A FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table.

The primary key and foreign key , however, must not be confused with each other as they have a completely different definition as well as usage. In this article i will first explain you the difference between primary key and foreign key in multiple bullet-points with real examples. Then I will explain the difference in to tabular format which gives the correct idea to the users. The difference between primary key and foreign key is that the primary key is used to identify the records in the table uniquely while the foreign key is used to connect two table together. The foreign key of a particular table is simply the primary key of some other table which is used as a reference key in the second table.

The autogenerated artificial column that we added as primary key in our example is also called as surrogate keys. Now we can use the s_id column in the table(ie: our primary key ), to uniquely identify one particular student as shown below. You can manually create an index on foreign key. We can have more than one foreign key in a table. It is perfectly fine to use a foreign key as the primary key if the table is connected by a one-to-one relationship, not a one-to-many relationship.

An example would be a list of homes on a real estate market. In a well-ordered database, there should be a primary key that uniquely identifies each record. Learn about difference between primary key and foreign key. The foreign key prevents invalid data from being inserted into the foreign key column. The values that you enter in the foreign key column have to be one of the values contained in the table that it points to.

A foreign key in one table points to primary key in another table. It allows the null value. On the other han the foreign key is just the primary key of another table. In a relational database, two tables can be related to each other e. Employee and Department, those cases, the primary key of one table becomes the foreign key of another table. Typically you take the primary key field from one table and insert it into the other table where it becomes a foreign key (it remains a primary key in the original table).

So, for example, OrderNo is the primary key of the ORDERS table below, and CustomerNo is a foreign key that points to the primary key in the CUSTOMERS table. Identity is used to make a column Auto Increment.