What is the difference between base and derived relation?
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A base relation is just a table that actually physically exists and is stored in the database.
This is in contrast to a view, which is derived from base relations but does not actually physically exist in the database - it only exists as a structural definition, pulling its data from a number of base relations.
A relational system has to provide a means for defining those base relations in the first place. In SQL, this task is performed by the CREATE TABLE statement (the SQL counterpart to a base relation being, of course, a base table, which is what CREATE TABLE creates). And base relations obviously need to be named
But certain derived relations, including in particular what are called views, are named too. A view (also known as a virtual relation) is a named relation whose value at any given time t is the result of evaluating a certain relational expression at that time t.
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