In the context of database management systems, a key is a unique identifier for a record in a table. Keys are used to ensure the integrity of the data in the database and to facilitate the efficient retrieval of data. There are several types of keys that are used in database design, including alternate, artificial, compound, and natural keys.

Alternate key: An alternate key is a unique identifier for a record in a table that is not the primary key. It can be used to uniquely identify a record, but it is not the primary key. For example, in a table of customers, the customer's email address could be an alternate key, since each customer is likely to have a unique email address.

Artificial key: An artificial key, also known as a surrogate key, is a unique identifier for a record that is created by the database management system rather than being derived from the data. This is typically done by assigning a unique number to each record as it is added to the table. Artificial keys are commonly used when there is no natural key that can be used to uniquely identify each record, or when the natural key is not appropriate for use as the primary key.

Compound key: A compound key is a key that is composed of two or more attributes of a record. For example, in a table of orders, a compound key might be composed of the order number and the customer ID, since each order is uniquely identified by both the order number and the customer who placed the order. Compound keys are often used when a single attribute is not sufficient to uniquely identify each record.

Natural key: A natural key is a unique identifier for a record that is derived from the data itself. For example, in a table of employees, the social security number could be used as the natural key, since each employee has a unique social security number. Natural keys are often preferred over artificial keys because they are more intuitive and can be more easily understood by users.

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In summary, alternate keys are unique identifiers for a record that are not the primary key, artificial keys are unique identifiers that are created by the database management system, compound keys are composed of two or more attributes of a record, and natural keys are unique identifiers that are derived from the data itself. Each type of key has its own advantages and disadvantages, and the choice of which type of key to use depends on the specific requirements of the database and the application that will be using it.