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A candidate key, or simply a key, of a relational database is any set of columns that have a unique combination of values in each row, with the additional constraint that removing any column could produce duplicate combinations of values. A candidate key is a minimal superkey, [1] i.e., a superkey that doesn't contain a smaller one. Therefore ...
Educational stages are subdivisions of formal learning, typically covering early childhood education, primary education, secondary education and tertiary education.The United Nations Educational, Scientific and Cultural Organization recognizes nine levels of education in its International Standard Classification of Education (ISCED) system (from Level 0 (pre-primary education) through Level 8 ...
Only one key within an entity is selected to be the primary key. This is the key that is allowed to migrate to other entities to define the relationships that exist among the entities. When the data model is instantiated into a physical database, it is the key that the system uses the most when accessing the table, or joining the tables ...
The primary–secondary quality distinction is a conceptual distinction in epistemology and metaphysics, concerning the nature of reality. It is most explicitly articulated by John Locke in his Essay concerning Human Understanding , but earlier thinkers such as Galileo and Descartes made similar distinctions.
K–12, [a] from kindergarten to 12th grade, is an English language expression that indicates the range of years of publicly supported primary and secondary education found in the United States and Canada, which is similar to publicly supported school grades before tertiary education in several other countries, such as Afghanistan, Australia, China, Ecuador, Egypt, India, Iran, the Philippines ...
A primary source can have all of these qualities, and a secondary source may have none of them. Deciding whether primary, secondary or tertiary sources are appropriate on any given occasion is a matter of good editorial judgment and common sense, not merely mindless, knee-jerk reactions to classification of a source as "primary" or "secondary".
In the relational model of databases, a primary key is a designated attribute that can reliably identify and distinguish between each individual record in a table.The database creator can choose an existing unique attribute or combination of attributes from the table (a natural key) to act as its primary key, or create a new attribute containing a unique ID that exists solely for this purpose ...
Despite the fundamental importance of using primary and secondary sources, the difference between these is sometimes unclear; the information required to assess whether a source, or part of a source, is primary or secondary is often contained in rather long, detailed, general guidelines which might also be covering Art, History, Law, etc.