a2zinterviews

51.	Define Unique Key?
	Unique key is a one or more column that must be unique for each row of the table. It is similar to primary key. Primary key column will not accept a null. Whereas the unique key column will accept a null values.

52.	Define Joins?
	A Join combines columns and data from two or more tables (and in rare cases, of one table with itself).

53.	Define Indexes?
	Index is a general term for an Oracle/SQL features used to primarily to speed execution and imposes uniqueness upon certain data. The most important of an index is to ensure uniqueness of rows and help in speedy retrieval of data.

54.	What are the difference between primary keys and foreign keys?
	The primary key is the columns used to uniquely identify each row of a table.A table can have only one primary key. Foreign keys are both a method of ensuring data integrity and a manifestation of the relationship between tables.

55.	Define Clusters?
	Clustering is a method of storing tables that are intimately related and often joined together into the same area on disk. A cluster contains one or more tables, which have one or more column in common among them.