What is a UNIQUE constraint?
In the realm of database design, a UNIQUE constraint is a rule or restriction applied to a column or a combination of columns in a database table. Its purpose is to ensure that the values in the specified column(s) are unique and distinct, meaning no duplicates are allowed.
When a UNIQUE constraint is defined on a column or a set of columns, the database management system (DBMS) automatically enforces this constraint whenever data is inserted or updated in the table. This prevents the possibility of having duplicate values in the specified column(s), helping to maintain data integrity and accuracy.
The UNIQUE constraint can be applied to a single column or multiple columns. In the case of a single column, each value must be unique within that column. For example, in a table storing employee information, the employee ID column can have a UNIQUE constraint, ensuring that each employee is assigned a unique ID.
In the case of multiple columns, the combination of values across those columns must be unique. For instance, in a table recording orders, a UNIQUE constraint can be applied to the combination of the customer ID and order ID columns, ensuring that each order is uniquely identified within the context of a specific customer.
The UNIQUE constraint provides a powerful tool to enforce uniqueness and integrity in a database table. By preventing duplicates, it allows for efficient data retrieval, prevents data inconsistencies, and helps avoid potential issues when performing updates or searches based on specific criteria.
It’s important to note that a UNIQUE constraint differs from a PRIMARY KEY constraint in that a PRIMARY KEY constraint not only enforces uniqueness but also declares a particular column or combination of columns as the primary identifier for a table. Additionally, a table can have multiple UNIQUE constraints, while it can have only one PRIMARY KEY constraint.
In summary, a UNIQUE constraint ensures that the values in a specified column or set of columns within a database table are unique and distinct. It plays a vital role in maintaining data integrity and allows for efficient data retrieval and manipulation.
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