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Primary Key in SQL Database: What is, Advantages & How to Choose

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24th May, 2021
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Primary Key in SQL Database: What is, Advantages & How to Choose

What is a Primary key?

Designing a new database in SQL comes with its share of inimitable choices. Identifying a suitable primary key is one of the most critical decisions you will have to make. The underlying objective of a primary key is simple and straightforward.

A primary key plays a two-pronged fundamental role. One, it helps to define a relationship. Two, it helps to implement this relationship. In other words, without a primary key, relationship databases wouldn’t exist. Understandably, the notion of a primary key sounds unusual.

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But what if we told you that you have been using primary keys in your everyday life without realizing it. At each waking moment in our day, we are literally surrounded by primary keys in a database world, and yet we take them for granted. For instance, consider your student ID. It is a classic example of the need for a primary key. Similarly, each employee has a unique company code, and each country in the world has a unique name and code. 

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What are the Advantages of a Primary key?

A primary key in SQL is defined as a singular value or a combination of a few values from a table. It distinctly represents each record in the table. Access to this value guarantees timely access to the placement of a related record in the table, in addition to other values from the same record. At this point, you might have an important question to raise. Is it mandatory to generate a primary key in each table? Absolutely not.

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SQL does not necessitate the creation of a primary key. However, the best and most efficient practices of database modelling strongly advise creating a primary key for every table in SQL. Besides being a common link field between tables, the advantages of a primary key are multifold.

The list below summarizes the main advantages of a primary key:

  1. Since the primary key serves as an index, speed-based database operations like sorting and searching for records becomes faster. 
  2. The primary key helps to quickly identify and find unique rows in a particular database table.
  3. In safe mode, only specific records can be uniquely identified using the primary key to ensure that updates and deletion only affect specific rows rather than larger volumes of data. 
  4. When used as foreign keys in other tables within the database, they help maintain referential integrity. 

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How to Choose a Primary key?

Tables comprise the main objects in an SQL database. These tables store data as records or rows. Therefore, to identify each row of a table, we need to find a column in the same table with a different input value for each row. For example, let us assume that we have a table with data on every US citizen. A column by the name ‘social_security_number’ would help distinguish one row from another in such a table.

Similarly, let us consider another example. Suppose we have a table of saving accounts at a private bank. The column by the name ‘account_number’ can be used to identify specific rows in this table uniquely. 

In both these examples, once you have identified a column with a different value for each row in the table, that is, ‘social_security_number’ and ‘account_number’, you can create your primary key. This primary key will be your main identifier for every row in the table, based on the specific column you have identified.

The mark of an exemplar database design is thus the primary key in SQL. Therefore, it should go without saying that a chosen primary key has to be 100% unique, and there must exist a unique value of the primary key for each row of data. Typically, individuals demonstrate a tendency to rely on the database management system itself to generate a unique identifier. 

The characteristics of a strong primary key are as follows:

  1. A strong primary key is usually short in length, compact and contains the fewest possible attributes. Long data types and compound keys add complexity to SQL. A primary key length should not exceed 900 bytes.
  2. Primary keys that are entirely numeric help in enhancing query performance. 
  3. Avoid the use of special characters or a combination of uppercase and lowercase letters. Avoid using text data types because it takes significantly longer for SQL to compare string values than numeric values.
  4. Avoid identifiable information from non-key columns in the same row such as zip codes, email addresses and social security numbers. If a primary key is based on real-world data, it may have to be changed at some point in the future, which is an inadvisable modelling practice. If primary keys are arbitrary identifiers, the data displayed to the user may change later, but the identifiers don’t have to. 
  5. The primary key value must remain stable and not be modified. 

It is imperative to note that primary keys employ a technical mechanism to ensure that every row has a unique, non-empty value in the primary key column. This implies that a record’s primary key value cannot be null. Additionally, suppose you attempt to insert a new with a value whose duplicate can be found in the primary key column. In that case, the new row’s insertion will be rejected by the primary key.

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Primary keys with More than One Column

In some instances, there does not exist a natural one-column primary key. When multiple columns are used to define a primary key, it is known as a multi-column or composite primary key. For example, let us assume that we have a table ‘reservation’ with columns, ‘customer_name’, ‘reservation_day’, ‘reservation_time’ and ‘number_of_people’.

Here, ‘customer_name’ cannot be used as the primary key alone because naturally, the customer might have more than one reservation for different days or even the same day. We, therefore, add ‘reservation_day’ and ‘reservation_time’ to the primary key to make the combination unique for each value. 

This consideration should not exceed more than 32 columns. The greater the number of columns, the greater will be the storage space requirement. The syntax to add a multicolumn primary key is different from adding a single-column primary key. 

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Summing Up

Have you ever called up customer care and been asked for your customer number? You might have even forgotten it, which is when they ask you for your postcode or telephone number.

Have you ever wondered why? Well, the short answer is the primary keys. The long answer is that the advantages of a primary key are the starting point of all database systems. Without primary keys, databases wouldn’t work as they work now, or to be precise, wouldn’t function at all. 

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Rohan Vats

Blog Author
Software Engineering Manager @ upGrad. Passionate about building large scale web apps with delightful experiences. In pursuit of transforming engineers into leaders.

Frequently Asked Questions (FAQs)

1What are primary keys in SQL?

Primary key is the field with unique values, which is used to define the identity of an entity. Once a primary key is defined, it cannot be deleted or modified. In fact, the primary key is a special characteristic of table, which becomes the basis for table’s relationships. Primary key is always defined in first column of a table, which cannot be altered. It holds a very important role in the logical and physical record structure of the data. Since primary key is used to uniquely identify the records, they are called as candidate keys.

2What are the characteristics of a primary key in SQL?

Primary key is a column or a group of columns that uniquely identify each record in a table. A table can have only one primary key. To maintain integrity, the primary key is always required. Primary key should be made up of combination of columns that are different from each other. If a primary key is made up of only one column, then it is known as a simple primary key. The database engine uses primary key to locate and update a record in a database. Primary key cannot be Null because Null means not defined or unknown. Primary key can be Single column or multi-column. Primary key values can be unique or non-unique. Primary key has the ability to be indexed. Primary key helps database engine to maintain table integrity. Primary key can be easily identified in the database.

3How to choose a primary key in SQL?

The primary key is the most important column in a table. These are the ids used by your apps to uniquely identify each record. The field you select as the primary key must be unique and not null. Primary keys are usually the best candidate columns for indexing. The primary key is used for: Unique reference for each record in the table, to look up the id of a record whose data you already have, to ensure that each row has a valid value in the primary key. To ensure data in the table remains valid and consistent, ensure that all primary key values are unique and that no primary key value is null.