{"id":80,"date":"2022-02-11T09:33:00","date_gmt":"2022-02-11T09:33:00","guid":{"rendered":"https:\/\/openquery.com\/?p=80"},"modified":"2022-02-28T13:55:57","modified_gmt":"2022-02-28T13:55:57","slug":"can-a-foreign-key-be-a-primary-key","status":"publish","type":"post","link":"https:\/\/openquery.com\/can-a-foreign-key-be-a-primary-key\/","title":{"rendered":"Can A Foreign Key Be A Primary Key?"},"content":{"rendered":"\n

Foreign Keys and Primary Keys are the perfect partners. You can think of them as the leads in your favorite buddy cop flick.<\/p>\n\n\n\n

They\u2019re both extremely different but in ways that wind up complementing each other, helping them to eventually foil the bad guys and save the day. But, instead of bad guys, Keys foil redundant data and empty table space!<\/p>\n\n\n\n

This tension between Foreign and Primary keys is what makes them effective database<\/a> tools, but are there any instances in which a Foreign Key can be either Foreign or Primary? Is such a thing even possible? Well, let\u2019s discuss the matter in more detail and find it!<\/p>\n\n\n\n

\"Can<\/figure><\/div>\n\n\n\n

What Are Foreign And Primary Keys?<\/strong><\/h2>\n\n\n\n

Put simply, Keys are tools used to link two tables together inside a database. The Foreign Key is the field in a (child) table that references a field in a (parent) table. The referenced field is known as the Primary Key<\/a>.<\/p>\n\n\n\n

These links are established for two core reasons:<\/p>\n\n\n\n

To Save Space<\/strong><\/h3>\n\n\n\n

Database objects (tables, indexes, etc) need to be as intuitive and efficient as possible, which means that redundant data needs to be kept to a minimum. Splitting data across two tables and linking them with Keys is a great way to do so.<\/p>\n\n\n\n

To Reduce Human Error During Data Queries And Manual Scanning<\/strong><\/h3>\n\n\n\n

Duplicate data in a single table can lead to human error during processing. For example, let\u2019s say that we owned an online store, and to keep track of orders, we created a table called tblOrders that contains the following columns: Customer Name, Customer Address, Order ID, and Order.<\/p>\n\n\n\n

If one of our customers keeps coming back to our store and placing orders, our table will be gathering the same name and address over and over again, which is confusing for our dispatch manager to deal with.<\/p>\n\n\n\n

Splitting the data into two tables and linking them with Keys eliminates these redundancies.<\/p>\n\n\n\n

What Seems To Be The Problem With A Hybrid Foreign\/Primary Key?<\/strong><\/h2>\n\n\n\n

The seemingly gaping flaw in our idea of a Key that can be either Foreign or Primary<\/a> is that they\u2019re fundamentally different. <\/p>\n\n\n\n

The golden rule of Foreign Keys is that the field is not unique; it can display duplicate information, while the golden rule of Primary Keys<\/a> is that they are unique, and cannot display duplicate information.<\/p>\n\n\n\n

So, if these Keys are completely contradictory in nature, how in the heck can a single field be either the Foreign or Primary Key? It just doesn\u2019t seem to make any sense, but that doesn\u2019t necessarily mean it\u2019s not possible.<\/p>\n\n\n\n

It\u2019s All About Relation Type<\/strong><\/h2>\n\n\n\n

Okay, so we\u2019ve established that the role of Foreign and Primary Keys is to form a relationship between two fields in a database, but here\u2019s the thing\u2026 the relationships between tables aren\u2019t always the same.<\/p>\n\n\n\n

There are three types of table relationship in total:<\/p>\n\n\n\n

One-to-One<\/strong><\/h3>\n\n\n\n

An individual record in one table is related to an individual record in another table.<\/p>\n\n\n\n

For example, a school\u2019s student record might be composed of two tables, one called Students<\/em>, and one called StdContact Details. <\/em>Both tables would contain the field, Student ID<\/em>, which would be linked to one another using Keys. As this relationship has no bearing on any other field outside the two that are linked, it\u2019s considered a one-to-one relationship.<\/p>\n\n\n\n

One-to-Many<\/strong><\/h3>\n\n\n\n

An individual record in one table is related to multiple records in another table.<\/p>\n\n\n\n

The link between the customer table and order table of a shop is the perfect example of a one-to-many relationship, as there can only be one customer per order, but there can be any number of orders.<\/p>\n\n\n\n

Many-to-Many<\/strong><\/h3>\n\n\n\n

Multiple records in one table are linked to multiple records in another table.<\/p>\n\n\n\n

An example of a many-to-many relationship would be two tables in a university database linking students to their chosen classes, as students can choose many classes, and classes have many students. <\/p>\n\n\n\n

However, technically speaking, many-to-many relationships are problematic in databases. Instead, a third object called a joining table will be created to sit between the two existing tables.<\/p>\n\n\n\n

Each of the original tables will then share a one-to-many relationship with the central joining table instead of a many-to-many relationship with each other.<\/p>\n\n\n\n

Only one of these relationship types is capable of supporting hybrid keys that can be both Foreign or<\/em> Primary. Any guesses which?<\/p>\n\n\n\n

The answer is\u2026 (drumroll, please)… one-to-one relationships!<\/strong><\/p>\n\n\n\n

Yep, one-to-one relationships are the only instance in database structure where the Foreign Key can also be the Primary Key.<\/p>\n\n\n\n

Now, bear in mind that when I say this, I don\u2019t mean that a Foreign Key can also be a Primary Key at the same time, just that it doesn\u2019t matter which of your two tables holds the Foreign or Primary Key.<\/p>\n\n\n\n

In a one-to-many relationship between tables, there is always a reference <\/em>key and a referenced <\/em>key, telling you which table has to hold the Foreign Key and which has to hold the Primary Key \u2014 you don\u2019t get the choice.<\/p>\n\n\n\n

A many-to-many relationship requires that third table, which has to harbor the corresponding Foreign Key to the two main tables\u2019 Primary Keys \u2014 again, you don\u2019t have a say in the matter.<\/p>\n\n\n\n

With a one-to-one relationship, on the other hand, each field is equally referenced and referencing, <\/em>so it doesn\u2019t matter how you allocate roles of Foreign and Primary Key between them. Switch them around all you want, and it won\u2019t affect the way the data behaves within the tables.<\/p>\n\n\n\n

Final Thoughts<\/strong><\/h2>\n\n\n\n

Well, we finally got down to the bottom of it \u2014 A Foreign Key can indeed be a Primary Key, but only in a very specific circumstance, and that\u2019s a one-to-one relationship between two tables. The question is\u2026 is it worth using a one-to-one pairing in the first place? Well\u2026 It all comes down to preference. <\/p>\n\n\n\n

Certain database designers feel that breaking a single table into two separate entities is inelegant, while others believe that it\u2019s much better to have two optimized tables than one table with lots of blank spaces. So, which do you prefer?<\/p>\n\n\n\n

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https:\/\/www.youtube.com\/watch?v=hjgJZtLgB4w<\/a>