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Old 02-15-2017, 07:01 AM   #1
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Thumbs up Define candidate key, alternate key, composite key.

A candidate key is one that can identify each row of a table uniquely. Generally a candidate key becomes the primary key of the table. If the table has more than one candidate key, one of them will become the primary key, and the rest are called alternate keys. A key formed by combining at least two or more columns is called composite key.

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Old 08-17-2017, 12:10 AM   #2
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Candidate keys are those keys which is candidate for primary key of a table. In simple words we can understand that such type of keys which full fill all the requirements of primary key which is not null and have unique records is a candidate for primary key. So thus type of key is known as candidate key

Primary keys are used to identify tables. There is only one primary key per table. In SQL Server when we create primary key to any table then a clustered index is automatically created to that column.

When we create keys on more than one column then that key is known as composite key. Like here we can take an example to understand this feature. I have a table Student which has two columns Sid and SrefNo and we make primary key on these two column. Then this key is known as composite key.
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