A data definition language creates, deletes, and modifies data structures. It applies to all languages, but is commonly used in SQL and XML. It must provide three functions: constructing data structures, deleting them selectively, and altering them without losing information.
A data definition language is a computer programming or scripting language that defines a data structure. By definition, these languages need to do three things: create, delete, and modify data structures. What they do outside of those three areas is irrelevant, as long as they do those three tasks. Any language can be a data definition language, but the term is most commonly applied to structured query language (SQL) and extensible markup language (XML) schemas.
When the term data definition language first came into use, it applied to a model made by the Conference for Data System Languages (Codasyl). The process defined two main areas of data structure development: the data definition language created the actual database structure, and the data manipulation language defined the methods for placing the data in the structure. Since then, these two terms have expanded and become generic terms for the processes they cover.
Generic terms now apply to all languages serving their original functions. Both SQL and XML perform all the required tasks and provide many features that were absent from the original model because they hadn’t been invented yet. Other languages also provide these features; they’re just used much less often.
To be a data definition language, the language must provide three functions. The first major function is the construction of data structures; basically, these are tables designed to hold specific groups of information. They often appear as a spreadsheet, holding pages of cross-referenced information. For example, the sheet might have a list of a company’s customers at the bottom one side and a list of available products at the top. The sheet would have a list of when those customers bought individual products in the table portion.
The next main function is the deletion of data structures. It’s not the same as deleting an entire database or file; it is a much more selective process. It can remove a specific page of information or an entire portion of a multidimensional array. In both cases, the data must be removed without affecting other data structures, even if they are all contained in the same file.
The last major function is the alteration of a data structure. This is a broad category that covers many situations. A table may have columns added or renamed, or an entire database may need to be split into two different databases. In any situation, they must be modified in such a way that no information is lost, destroyed or created in the process. This prevents anomalous information from entering the data system.
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