Unique Constraint


A “Unique Constraint” in technology, specifically in database management, refers to a rule applied to a column or set of columns in a database table that prevents duplicate values. It ensures that each record within the column or set of columns is unique, thus maintaining data integrity. Failure to adhere to this constraint results in an error since it prohibits multiple rows from having the same value in certain column(s).


The phonetics of the keyword “Unique Constraint” would be:yo͞o-nēk kən-ˈstrānt

Key Takeaways

  1. Preventing Duplicates: The main purpose of a unique constraint is to ensure that each entry in a particular column (or combination of columns) of a database table is unique. This provides a way to prevent duplicate entries in the table that might compromise the integrity of the data.
  2. Uniqueness across Rows: A unique constraint applies across the rows of a table – it means that the column’s entries, combined over all the rows, need to be unique. For a single column, this is straightforward; for multiple columns, the constraint means that every column tuple must be unique.
  3. Support for NULL Values: In most databases, the unique constraint allows for one null entry, because null is not considered an actual value in terms of unique constraints. Thus, multiple null entries do not violate the unique constraint. However, this may vary across different database systems.


Unique Constraint is a vital technical concept in the field of database management, particularly in ensuring data integrity and accuracy. This term refers to a rule applied to restrict duplicate entries in a specific column or a combination of columns. Essentially, it guarantees that all values in a column, or set of columns, are unique across the rows within a database table. It aids in avoiding the repetition of data and ensures that each entry is distinct and accurate. This is crucial as it not only improves the reliability of data and the efficiency of data retrieval operations, but it also supports the database in delivering accurate outputs for analyses and reports.


A unique constraint is a rule applied in databases to ensure that all data within a specific column, or a combination of multiple columns, are unique across all rows. The main purpose of unique constraints is to maintain data integrity and accuracy within a database. Unique constraints prevent redundant or duplicate records in a table, thereby maximizing the efficiency of the database. For instance, in a school’s student database, a unique constraint might be applied to a student’s identification number to ensure that no two students have the same number.The benefit of using a unique constraint is that it assists in preserving the reliability of the data and strengthens data validation rules in a database. For instance, in an email distribution system, the unique constraint would prevent the same email address from being added to the system more than once, thus stopping any accidental spamming of a single user. Implementing a unique constraint is a critical practice to avoid potential anomalies in data, enhance performance, and improve data consistency in a database. It is a tool for implementing business rules and contributing to the overall data quality management process.


A unique constraint is a rule that is applied to maintain data integrity by preventing duplicate entries in a specific column or set of columns in a database table.Here are three real-world examples of how unique constraints are applied:1. **Social Security Numbers (SSNs) in a government database**: The US Social Security Number (SSN) is unique to each individual. If the government has a database to store citizen data, the SSN field/column will be set to have a unique constraint to avoid duplication of records.2. **Email addresses in a user login system**: If you are building a system where users need to login, typically, you would want email addresses to be unique. This ensures that users can’t register multiple accounts using the same email address. In this case, a unique constraint can be applied to the email column in the database ensuring that each email address can only be used once.3. **Product Codes in an Inventory system**: In a retail organization’s inventory database, each product has a unique product code, also known as SKU (Stock Keeping Unit). The product code field will have a unique constraint to ensure that there aren’t any duplicate entries causing confusion about the actual quantity of the stock.

Frequently Asked Questions(FAQ)

**Q1: What is a Unique Constraint?**A: A Unique Constraint is a rule which is applied to a field or a column in a relational database to prevent duplicate values from being entered into the same column or field. **Q2: Why are Unique Constraints used in a database management system?** A: Unique Constraints are used mainly to ensure data integrity in the database. They prevent duplicate data in the specific columns where the constraints have been set.**Q3: Can multiple Unique Constraints be set on a single table?**A: Yes, one can set multiple Unique Constraints on a single table but they need to be on different columns.**Q4: What happens when a unique constraint is violated?**A: If a unique constraint is violated, the Relational Database Management System (RDBMS) will reject the action and return an error.**Q5: Can a Unique Constraint be applied on a column which already has duplicates?**A: No, a Unique Constraint cannot be applied on a column if it already has duplicate data. The duplicates need to be removed first.**Q6: Does a Unique Constraint allow null values?**A: Yes, a Unique Constraint generally allows for one null value because null is technically not a value and so it doesn’t violate the condition of uniqueness.**Q7: How can we identify a Unique Constraint in a database table?**A: Unique Constraints can be identified by viewing the table’s schema or design layout. In programs like SQL Server Management Studio or Oracle SQL Developer, unique constraints are generally listed as part of the table’s properties.**Q8: Is Unique Constraint the same as Primary Key?**A: No, while both ensure uniqueness of the column, the primary key doesn’t allow null values whereas usually, the unique constraint does.**Q9: How can a Unique Constraint be removed?**A: To remove a Unique Constraint, you can use SQL commands. The command usually follows this syntax: “ALTER TABLE table_name DROP CONSTRAINT constraint_name;”**Q10: Can a Unique Constraint be modified?**A: No, a Unique Constraint cannot be modified directly. You need to drop the constraint first and then recreate it.

Related Tech Terms

  • Primary Key
  • Foreign Key
  • Data Integrity
  • Relational Database
  • Schema Design

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