What is a unique key?

A unique key is a database constraint that ensures that the values in one or more columns in a table are unique across all the rows in the table. It is similar to a primary key, but unlike a primary key, a table can have multiple unique keys.

Like a primary key, a unique key is used to ensure data integrity and consistency in a table. It helps to prevent duplicate data from being inserted into a table, and it also makes it easier to search for and retrieve specific records from the table.

Unique keys are often used for fields that are important for data integrity but are not used as the primary means of identifying records in the table. For example, a table of customers may have a primary key consisting of a unique customer ID, but it may also have a unique key on the email address field to ensure that no two customers have the same email address.

In addition to preventing duplicate data, unique keys can also be used to speed up searches and other database operations. For example, a query that searches for all records with a specific value in a unique key field can be optimized by using an index on that field.