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an arrow from an entity set to a relationship set indicates a key constraint, i.e. injectivity: each entity of the entity set can participate in at most one relationship in the relationship set; a thick line indicates both, i.e. bijectivity: each entity in the entity set is involved in exactly one relationship.
For example, think of A as Authors, and B as Books. An Author can write several Books, and a Book can be written by several Authors. In a relational database management system, such relationships are usually implemented by means of an associative table (also known as join table, junction table or cross-reference table), say, AB with two one-to-many relationships A → AB and B → AB.
Thus, the "relation" in "relational database" refers to the various tables in the database; a relation is a set of tuples. The columns enumerate the various attributes of the entity (the employee's name, address or phone number, for example), and a row is an actual instance of the entity (a specific employee) that is represented by the relation.
Overview of a data-modeling context: Data model is based on Data, Data relationship, Data semantic and Data constraint. A data model provides the details of information to be stored, and is of primary use when the final product is the generation of computer software code for an application or the preparation of a functional specification to aid a computer software make-or-buy decision.
A simple Entity–Component–System layout. Entity–component–system (ECS) is a software architectural pattern mostly used in video game development for the representation of game world objects. An ECS comprises entities composed from components of data, with systems which operate on the components.
A relational database (RDB [1]) is a database based on the relational model of data, as proposed by E. F. Codd in 1970. [2]A Relational Database Management System (RDBMS) is a type of database management system that stores data in a structured format using rows and columns.
A relationship can be specified to take an Action when some Operation is done on a related entity. For example, on deleting an entity that forms the part of a relation (the OnDelete operation) the actions that can be taken are: [38] Cascade, which instructs to delete the relationship instance and all associated entity instances. None.
The result is sometimes referred to as a "derived" relation when the operands are relations assigned to database variables. A view is defined by giving a name to such an expression, such that the name can subsequently be used as a variable name. (Note that the expression must then mention at least one base relation variable.)