Define candidate key, alternate key, composite key?
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Answer Posted / manumole
Candidate key:
Candidate keys for a table includes the set of columns
which satisfies the criteria of a primary key.
Primary key : a column/combination of columns which
uniquely identifies a row and whose values are not null.
Clustered Key:when a combination of columns becomes a
primary key it is called as clustered key.
Alternate keys:the set of columns which is avialble in the
candidate keys and are not selected as primary key.
Eg:Customer table
candidate keys:{ CustomerID, MobNo(defined as not null) }
primary key: if Db designer selected as CustomerID,then
Alternate key is MobNo
Clustered key:if both combination CustomerID- MobNo is
taken as primary key,it is nothing but a clustered key
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