A tree data structure can be defined recursively (locally) as a collection of nodes (starting at a root node), where each node is a data structure composed of a value, thus as a list of references to nodes (the children), with the constraints that no reference is duplicated and that none points to the root. Alternatively, a tree can be defined abstractly as a whole (globally) as an ordered tree, with a value assigned to each node. Both of these perspectives are useful: while a tree can be analyzed mathematically as a whole, when it is actually represented as a data structure, it is usually represented and used separately by node (rather than as a list of nodes and a edge adjacency list between nodes). , as we can represent a digraph for example). Say no to plagiarism. Get a tailor-made essay on “Why violent video games should not be banned”? Get an original essay For example, looking at a tree as a whole, we might talk about the parent node of a given node, but usually as a given structure a given node only contains the list of its children, but does not contain a reference to its parent (if any). In the context of relational databases, a foreign key is a field (or collection of fields) in a table that uniquely identifies a row in another table or the same table. In simpler terms, the foreign key is defined in a second table, but it refers to the primary key or a unique key in the first table. For example, a table called Employee has a primary key called Employee_id. Another table called Employee Details has a foreign key that references Employee_id to uniquely identify the relationship between the two tables. The table containing the foreign key is called a child table and the table containing the candidate key is called a referenced or parent table. In relational database modeling and implementation, a unique key is a set of zero or more attributes, the value(s) of which are guaranteed to be unique for each tuple (row) in a relationship. Keep in mind: this is just an example. .Get a custom paper now from our expert writers.Get a custom essayThe value or combination of key attribute values ​​unique to one tuple cannot be duplicated for another tuple in that relationship. When multiple columns are combined to form a single key, their combination is used to access each row and maintain its uniqueness. Column values ​​are not combined, they are compared using their data types. Defining a foreign key In the ISO SQL standard, foreign keys are defined via a FOREIGN KEY constraint. The syntax for adding such a constraint to an existing table is defined in SQL:2003 as shown below.