An XML bomb is a piece of XML code that is syntactically valid and correct but can cause a program that compiles or runs it to crash or hang. An XML bomb can be used to test the security level of a server. In an HTML code, an XML code is either parsed internally or referenced as an external file that is sent to a server. Typically, a normal server...
The relational model is the conceptual basis of relational databases. Proposed by E.F. Codd in 1969, it is a method of structuring data using relations, which are grid-like mathematical structures consisting of columns and rows. Codd proposed the relational model for IBM, but he had no idea how extremely vital and influential his work would become as the basis of relational databases. Most of us are very familiar with the physical manifestation of a relation in a database - it is called a table.
Although the relational model borrows heavily from mathematics and uses mathematical terms such as domains, unions and ranges, the features and conditions it describes are easy to define using simple English.
In the relational model, all data must be stored in relations (tables), and each relation consists of rows and columns. Each relation must have a header and body. The header is simply the list of columns in the relation. The body is the set of data that actually populates the relation, organized into rows. You can extrapolate that the junction of one column and one row will result in a unique value - this value is called a tuple.
The second major characteristic of the relational model is the usage of keys. These are specially designated columns within a relation, used to order data or relate data to other relations. One of the most important keys is the primary key, which is used to uniquely identify each row of data. To make querying for data easier, most relational databases go further and physically order the data by the primary key. Foreign keys relate data in one relation to the primary key of another relation.
Besides defining how the data are to be structured as discussed above, the relational model also lays down a set of rules to enforce data integrity, known as integrity constraints. It also defines how the data are to be manipulated (relational calculus). In addition, the model defines a special feature termed normalization to ensure efficient data storage.
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