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A primary key is a special relational database table column (or combination of columns) designated to uniquely identify each table record.
A primary key is used as a unique identifier to quickly parse data within the table. A table cannot have more than one primary key.
A primary key’s main features are:
- It must contain a unique value for each row of data.
- It cannot contain null values.
- Every row must have a primary key value.
A primary key might use one or more fields already present in the underlying data model, or a specific extra field can be created to be the primary key.
Techopedia Explains Primary Key
The primary key concept is critical to an efficient relational database. Without the primary key and closely related foreign key concepts, relational databases would not work.
In fact, since a table can easily contain thousands of records (including duplicates), a primary key is necessary to ensure that a table record can always be uniquely identified.
All keys that come from real-world observables and attributes are called natural primary keys, as opposed to surrogate primary keys that are, instead, arbitrarily assigned to each record.
Almost all individuals deal with natural primary keys frequently but unknowingly in everyday life.
For example, students are routinely assigned unique identification (ID) numbers, and all U.S. citizens have government-assigned and uniquely identifiable Social Security numbers. Street addresses or driver license numbers are examples of primary keys used to uniquely identify (respectively) locations or cars.
As another example, a database must hold all of the data stored by a commercial bank. Two of the database tables include the CUSTOMER_MASTER, which stores basic and static customer data (name, date of birth, address, Social Security number, etc.) and the ACCOUNTS_MASTER, which stores various bank account data (account creation date, account type, withdrawal limits or corresponding account information, etc.).
To uniquely identify customers, a column or combination of columns is selected to guarantee that two customers never have the same unique value. Thus, certain columns are immediately eliminated, e.g., surname and date of birth.
A good primary key candidate is the column that is designated to hold Social Security numbers. However, some account holders may not have Social Security numbers, so this column’s candidacy is eliminated.
The next logical option is to use a combination of columns, such as adding the surname to the date of birth to the email address, resulting in a long and cumbersome primary key.
The best option is to create a separate primary key in a new column named CUSTOMER_ID. Then, the database automatically generates a unique number each time a customer is added, guaranteeing unique identification.
As this key is created, the column is designated as the primary key within the SQL script that creates the table, and all null values are automatically rejected.
The account number associated with each CUSTOMER_ID allows for the secure handling of customer queries and quick search times (as with any indexed table.)
For example, a customer may be asked to provide his surname when conducting a bank query. A common surname (such as Smith) query is likely to return multiple results.
When querying data, utilizing the primary key uniqueness feature guarantees one result.
