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Simond Gear
Simond Gear

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Posted on    February-28-2016 10:04 PM

 Database Database  Keys  Database Design 

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Please give me a exact difference between among different types  of keys in sql such as Primary Key,Foriegn Key etc.
Thank you.

Anupam Mishra

Total Post:135

Posted on    February-28-2016 11:48 PM

Hi Simond,
Their is basically following keys in SQL:

1. Candidate Key
2. Primary Key
3. Foriegn Key
4. Alternate Key
5. Composite Key

Here, we are explaining a simpler way to each key :

Candidate Key: 

Candidate keys are those keys which is candidate for primary key of a table. In simpler 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 .

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

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.

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

Composite Key:

When we create keys on more than one column then that key is known as composite key. I have a table Employee which has two columns Empid and EmpReferenceNo and we make primary key on these two column. Then this key is known as composite key:

For More details with example ,Please click HERE

Modified On Feb-28-2016 11:49:17 PM

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