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What is a foreign key, and what is it used for?
A foreign key is used to establish relationships between relations in the relational model. Technically, a foreign key is a column (or columns) appearing in one relation that is (are) the primary key of another table. Although there might be exceptions, the values in the foreign key columns usually must correspond to values existing in the set of primary key values. This correspondence necessity is created in a database using a referential integrity constraint on the foreign key.
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What is substitutability? Any method of a class-say A can equally well be adjured with any object belonging to any subclasses B of A. This characteristic leads to code reuse, a
This assignment concerns the Research database. The database ER model is shown above, where attributes are omitted for clarity. The relational model for the database is: Departm
System error :These contain errors in database system or the OS, example, deadlocks.Such errors are fairly hard to detect and need reprogramming the erroneous components of the sys
what is physical data structure
does tables are called relations
What do you mean by Correlated subquery? Subqueries, or nested queries, are used to bring back a set of rows to be used by the parent query. Depending on how the subquery is writ
What is the use of Union and intersection operation? Union: The result of this operation contains all tuples that are either in r1 or in r2 or in both r1 and r2.Duplicate tuple
What does Aggregation mean?
10A.53 Code an Oracle Database trigger to enforce the constraint that an employee can never change his or her department. 10A.54 Code an Oracle Database trigger to allow the delet
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