Both primary and unique keys are responsible for making relational database management systems function in their proper way. Through this article, we aim to help our readers know more about what is the primary key, what is a unique key, the main difference between primary key and unique key, the difference between unique and primary key in a tabulated format, etc. Read on for more.
Primary Key vs. Unique Key
|Basis of Difference
|By default, primary keys cannot accept NULL value
|Unique keys can allow only one NULL value
|By default, the primary key is a clustered index, and the data present in the table of the RDBMS is organized physically in sequence and as a unique clustered index
|By default, the unique key is a non-clustered, unique index
|Only one primary key is possible in a table
|More than one unique key can be present in a table
|Used for serving as a unique identifier for individual rows in a table.
|Along with the primary key, this key uniquely determines an individual row, which is not defined in any way by the primary key.
What is Primary key?
Before we delve into the difference between a unique key and primary key, it is essential to know the definition of a primary key in DBMS or a database management system. A primary key is used as a necessary means of identifying each record in a given table. It helps in identifying a row by using the data present within the row. In general, the primary key is based on any one or more of the given columns, for instance, the first and last name. In many other DBMS designs, the primary key happens to be an auto-generated number belonging to an identity column.
As you understand the features and functions of unique key and primary key in-depth, you will find all relational database management systems supporting primary keys.
For any table to be reckoned as a relational table, as in its first standard form, the table must comprise of a primary key. In other words, the primary key is an essential and necessary part of any RDBMS.
In context to SQLServer, when programmers refer to the primary key, they term it as the PRIMARY KEY constraint. The following characteristics are to be noted in a primary key:
- There can be just one primary key in any table.
- The primary key comprises either one or more columns.
- It is useful for enforcing the entity integrity of any table.
- The columns defined in the table must have the default value of NOT NULL.
- The primary key is used for identifying a row uniquely.
- A primary key will result in unique, CLUSTERED indexes by default.
In a nutshell, between the unique key and primary key, the primary key happens to be the main key of a given table and is used for identifying each row.
When do you use a primary key?
Always. A primary key refers to a unique table column (or a combination of columns) in a relational database. It is defined and designated to identify all available table records in the database uniquely. As the primary contains a unique value for every row of data, null values cannot be held by the primary key. A primary key example is given below:
Say, there exists a Student table in your RDBMS with attributes like Roll_number, Name, Year, Contact_number, Aadhar_ID.
Table Name: Student
In the above table, the attribute named Roll_number can never have a null or identical value as each enrolled student will be provided with a roll number at the time of his/ her admission to the university. More so, no two students can possess the same roll number as the authorities will uniquely identify each entity. So, the Roll_number attribute serves as the primary key in this table.
A primary key is capable of being referenced by a foreign key. It builds a unique clustered index that is related to the table. The data rows are sorted in a clustered index and stored in views or a table based upon the fundamental values. As the data rows can be classified in just one order in any table, there can be only one clustered index in any table.
What is Unique Key?
Similar to the primary key in DBMS, the unique key constraint is also used for identifying an individual tuple in any relationship in a unique manner. Unique key constraints comprise a set of either one or multiple fields/columns of any table that identifies a record uniquely (in a given database table). A unique key is capable of having only one null value. Just like the primary key, it cannot have any duplicate values. In other words, just like the primary key, a unique key also offers a guarantee for the uniqueness of columns or multiple columns. There may exist various unique keys in any table, but there will be only one primary key constraint for any given table.
In SQL Server, the following characteristics are depicted by unique keys:
- Multiple unique keys can be defined on a table.
- By default, unique keys will result in unique and NONCLUSTERED indexes.
- A unique key is usually made up of one or more columns in a given table.
- A column may have a NULL value, but only one NULL per column is permissible.
- It is possible to reference a unique constraint by a Foreign Key Constraint.
When do you use a unique key?
Unique key constraints are put in use to ensure that the data contained in any column is not duplicated in any two rows present in the database. This is the right way of providing data validation. Only one row in the relational database is permitted to have a null value for the unique key constraint. In the example cited above, the column Aadhar_ID can be assigned as the unique key wherein each entry will not be duplicate but unique and as appointed by the Government as a unique identification number. In case a student has migrated from another country, then the value of Aadhar_ID will be NULL in his/ her case. This is permissible as only one NULL can be included in the unique constraint. This is an essential difference between primary and unique keys.
Unique constraints are designed to be referenced by any foreign key. They can be used in case constraints have to be enforced on one or more columns that are not the primary key. Unlike in the case of primary keys, unique constraints are known to generate the non-clustered index that has a distinct structure from data rows. The individual key-value entries in non-clustered indexes point to the data row containing the key-value, hence the use of pointers.
Key Difference between Unique key and Primary key
- It is commonplace for RDBMS users to ask, “What is the difference between primary key and a unique key?” The primary key and unique keys have their own set of features and functionality that differ in some instances. The UNIQUE Key and PRIMARY Key constraints are both similar in the sense that they are used for enforcing the uniqueness of the particular column that they are defined on.
- Every table in the database structure is capable of having just a single primary key. On the other hand, there may exist multiple unique constraints on any given table.
- The attribute declared as a primary key is not equipped to accept NULL values. In contrast, the attribute declared as the unique constraint is capable of receiving one NULL value.
- When a primary key is defined, a clustered index is automatically created by default. Its definition results in the data present in the database table is physically organized, and in sequence, in the clustered index. However, the definition of a unique key ends up generating a non-clustered index.
Both Primary key and Unique key in SQL serve as unique identifiers for the rows belonging to the tables in a database. They identify the unique values in any column or set of columns. The significant point about unique vs. primary key is that while a table has a maximum of one primary key, multiple unique keys can be assigned to the same.
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