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What is difference between primary key and alternate key and candidate key??

8th Feb 2017, 6:16 PM
Joshi Chand
Joshi Chand - avatar
6 Antworten
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Candidate Key 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. Every table must have at least one candidate key but at the same time can have several.
9th Feb 2017, 9:29 AM
Akwin Lopez
Akwin Lopez - avatar
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Example: STUDENT {SID, FNAME, LNAME, COURSEID} Here in STUDENT table keys are: Candidate keys are SID or FNAME+LAME Primary Key: SID Foreign Key: COURSEID Alternate Key: FNAME+LAME Composite Key: FNAME+LAME
9th Feb 2017, 9:28 AM
Akwin Lopez
Akwin Lopez - avatar
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Primary Key Such type of candidate key which is chosen as a primary key for table is known as primary 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.
9th Feb 2017, 9:29 AM
Akwin Lopez
Akwin Lopez - avatar
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Foreign Key Foreign key are those keys which is used to define relationship between two tables. When we want to implement relationship between two tables then we use concept of foreign key. It is also known as referential integrity. We can create more than one foreign key per table. foreign key is generally a primary key from one table that appears as a field in another where the first table has a relationship to the second. In other words, if we had a table A with a primary key X that linked to a table B where X was a field in B, then X would be a foreign key in B.
9th Feb 2017, 9:29 AM
Akwin Lopez
Akwin Lopez - avatar
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Alternate Key If any table have more than one candidate key, then after choosing primary key from those candidate key, rest of candidate keys are known as an alternate key of that table. Like here we can take a very simple example to understand the concept of alternate key. Suppose we have a table named Employee which has two columns EmpID and EmpMail, both have not null attributes and unique value. So both columns are treated as candidate key. Now we make EmpID as a primary key to that table then EmpMail is known as alternate key.
9th Feb 2017, 9:29 AM
Akwin Lopez
Akwin Lopez - avatar
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Composite Key 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.
9th Feb 2017, 9:29 AM
Akwin Lopez
Akwin Lopez - avatar