In computer programming, create, read, update, and delete (as an acronym CRUD) are the four basic functions of persistent storage. Alternate words are sometimes used when defining the four basic functions of CRUD, such as retrieve instead of read, modify instead of update, or destroy instead of delete. CRUD is also sometimes used to describe user interface conventions that facilitate viewing, searching, and changing information; often using computer-based forms and reports. The term was likely first popularized by James Martin in his 1983 book Managing the Data-base Environment. The acronym may be extended to CRUDL to cover listing of large data sets which bring additional complexity such as pagination when the data sets are too large to hold easily in memory.
The acronym CRUD refers to all of the major functions that are implemented in relational database applications. Each letter in the acronym can map to a standard Structured Query Language (SQL) statement, Hypertext Transfer Protocol (HTTP) method (this is typically used to build RESTful APIs) or Data Distribution Service (DDS) operation:
|Create||INSERT||PUT / POST||write|
|Read (Retrieve)||SELECT||GET||read / take|
|Update (Modify)||UPDATE||PUT / POST / PATCH||write|
The comparison of the database oriented CRUD operations to the HTTP methods has some flaws. Strictly speaking, both PUT and POST can create resources; the key difference is that POST leaves it up to the server to decide at what URI to make the new resource available, while PUT dictates what URI to use; URIs as a concept do not align neatly with CRUD. The significant point about PUT is that it will replace whatever resource the URI was previously referring to with a brand new version, hence the PUT method being listed for Update as well. PUT is a 'replace' operation, which one could argue is not 'update'.
Although a relational database provides a common persistence layer in software applications, numerous other persistence layers exist. CRUD functionality can for example be implemented with object databases, XML databases, flat text files, or custom file formats.
CRUD is also relevant at the user interface level of most applications. For example, in address book software, the basic storage unit is an individual contact entry. As a bare minimum, the software must allow the user to
Without at least these four operations, the software cannot be considered complete. Because these operations are so fundamental, they are often documented and described under one comprehensive heading, such as "contact management", "content management" or "contact maintenance" (or "document management" in general, depending on the basic storage unit for the particular application).
Other variations of CRUD include:
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