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A unique constraint is a database constraint that ensures that the data stored in a column or a set of columns is unique. It is used to prevent two records from having identical values in a column. Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns.
Unique constraints are used to enforce the uniqueness of values in a column or in a set of columns in a particular table – that is, for columns that are not included in the table’s primary key set. A constraint is a rule that makes a column fulfill a particular condition (i.e. it prevents the user from entering the wrong type of data). A UNIQUE constraint dictates that a column must not store duplicate values.
When you create a unique constraint on one or more columns, you are telling the database management system (DBMS) to ensure that no two rows have the same value for those columns. If you try to insert or update data that violates this constraint, the DBMS will return an error.
Unique constraints can be used to enforce business rules or other requirements that dictate that certain data must be unique. For example, you might use a unique constraint to ensure that each customer has only one account.
Here are some reasons why you might want to use unique constraints:
– To ensure that each row in a table is unique.
– To enforce business rules or other requirements that dictate that certain data must be unique.
– To improve performance by allowing the DBMS to use indexes more efficiently.
– To simplify queries by allowing you to use simpler WHERE clauses.
Here are some websites where you can learn more about unique constraints:
– [W3Schools](https://www.w3schools.com/sqL/sql_unique.asp)
– [LearnSQL.com](https://learnsql.com/blog/unique-constraint-in-sql/)
– [Tutorialspoint](https://www.tutorialspoint.com/sql/sql-unique.htm)
– [Microsoft SQL Server](https://learn.microsoft.com/en-us/sql/relational-databases/tables/unique-constraints-and-check-constraints?view=sql-server-ver16)
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