A tree data structure can be defined recursively (locally) as a collection of nodes (starting from a root node), where each node is a data structure consisting of a value, along with a list of references to nodes (the children), with the constraint that no references are duplicated and none point to the root. Alternatively, a tree can be abstractly defined 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 actually represented as a data structure it is usually represented and worked on separately by node (rather than as a list of nodes and a list of adjacencies of edges between nodes , how one might represent a digraph, for example). Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essayFor example, looking at a tree as a whole, we can talk about the parent node of a given node, but in general as of a given structure a given node contains only the list of its children, but does not contain a reference to the his parent (if present). 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 in the same table. In simpler words, the foreign key is defined in a second table, but refers to the primary key or a unique key in the first table. For example, a table named Employee has a primary key named Employee_id. Another table called Employee Details has a foreign key that references Employee_id to uniquely identify the relationship between both tables. The table containing the foreign key is called a child table, while the table containing the candidate key is called a reference table or parent table. In relational database modeling and implementation, a unique key is a set of zero or more attributes, the values ​​of which are guaranteed to be unique for each tuple (row) in a relation. Please note: This is just an example Get a custom article from our expert writers now. Get a Custom Essay The value or combination of key attribute values ​​unique to any tuple cannot be duplicated for any other tuple in that relationship. When multiple columns are combined to form a unique key, their combination is used to access each row and maintain uniqueness. Column values ​​are not combined, they are compared using their data types. Foreign Key Definition 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.