A unique constraint is one of the essential features in database design and data management systems. When you want to store data in such a way that each record must be unique, the use of the concept of uniqueness is necessary. For instance, suppose you have a number of mobile phones and you do not want any numbers to be repeated; in such cases, a unique key comes to your aid.
The term uniqueness means that no two rows can have the same value in one or more columns. This issue can prevent the influx of duplicate data and the creation of a data duplication can be very effective. In SQL, you can use the unique code to determine a column or a set of columns that must contain unique values.
By using the unique code, not only is it guaranteed that the stored data will be unique, but it also prevents the entry of erroneous data that can exist due to duplicate entries, which can lead to a decrease in the quality of data and the performance of your data repository.
Just like a primary key, a unique key can be a combination of several columns or a single column. However, the main difference between these two is that a table can only have one primary key but can have multiple unique keys.
Practical Use of Unique in SQL
CREATE TABLE Customers (
CustomerID int NOT NULL,
Email nvarchar(255) UNIQUE,
PhoneNumber varchar(15),
PRIMARY KEY (CustomerID)
);
The above example shows how you can add a unique key in the CREATE TABLE statement. Here, the column Email is defined as a unique column. This means that no two records can have the same email.
On the other hand, the column CustomerID is specified as the primary key, which includes unique values by default.
The column PhoneNumber can have duplicate values; since it is not defined as a unique key, it can accept duplicate values in this field.
By specifying the correct and precise definitions of unique keys, you can ensure a higher quality of data storage with integrity and efficiency in your database design.