Difference Between Primary Key and Unique Key
Primary and unique keys are both necessary for relational database management systems to work properly. We hope that by reading this post, our readers will have a better understanding of what a primary key is, what a unique key is, the major distinction between primary and unique keys, and the difference between unique and primary keys in a tabular style, and so on.
What Is a Primary Key?
Before we look at the differences between a unique key and the main key, it's important to understand what a primary key is in a database management system (DBMS). A primary key is used to uniquely identify each entry in a database table. It aids in the identification of a row by using the data included inside the row. The main key is usually based on one or more of the provided columns, such as the first and last name. The main key in many other DBMS systems is an auto-generated integer corresponding to an identification column.
All relational database management systems support primary keys after you learn the capabilities and operations of unique and primary keys in full.
A primary key is required for every table to be classified as a relational table, as it was in its original form. In other words, the primary key is a critical component of every relational database management system.
When programmers refer to the main key in SQLServer, they refer to it as the PRIMARY KEY constraint. In a main key, the following qualities should be noted:
- In every table, there can only be one primary key.
- Either one or more columns make up the main key.
- It may be used to ensure that any table's entity integrity is maintained.
- The default value for the table's declared columns must be NOT NULL.
- The main key is used to uniquely identify each row.
By default, a primary key generates unique, CLUSTERED indexes.
In a nutshell, the primary key is the principal key of a table and is used to identify each row between the unique key and the primary key.
What Is a Unique Key?
The unique key constraint, like the primary key in DBMS, is used to identify an individual tuple in any connection in a unique way. Unique key constraints are a collection of one or more fields/columns in any table that uniquely identify a record (in a given database table). Only one null value is possible for a unique key. It cannot have any duplicate values, much as the main key. To put it another way, a unique key, like the main key, ensures the uniqueness of columns or multiple columns. There may be several unique keys in a table, but each table will have only one main key constraint.
Unique keys in SQL Server represent the following characteristics:
- On a table, many unique keys may be specified.
- Unique keys will result in unique and NONCLUSTERED indexes by default.
- A unique key in a table is generally made up of one or more columns.
- A NULL value is allowed in a column, however, only one NULL is allowed per column.
- A Foreign Key Constraint may be used to refer to a unique constraint.
Comparison Table Between Primary Key and Unique Key
| Primary Key |
Unique Key |
- In a database table, a primary key is used to uniquely identify a record or row.
|
- A unique key is used to identify all conceivable rows in a table, not just the ones that are presently present.
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- NULL values are not accepted.
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- In a table, it can only take one NULL value.
|
- By default, it is a clustered index, which implies that data is structured in a clustered index sequence
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- By default, it's a single non-clustered index.
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- In a table, there can only be one main key.
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- There may be numerous unique keys in a table.
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- The PRIMARY KEY constraint is used to specify the primary key.
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- A UNIQUE constraint is used to indicate a unique key.
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- In a table, it's used to identify a row.
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- To avoid duplicate values in a column, use this function.
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- The values of primary keys cannot be altered or removed.
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- Modifications to unique key values are possible.
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Conclusion
In SQL, both the primary key and the unique key serve as unique identifiers for the rows in a database's tables. They recognize the distinct values in any column or combination of columns. The difference between unique and primary keys is that although a table can only have one main key, it may have numerous unique keys.
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