Different types of SQL keys and their usage

Asked By Barb Brennan 20 points N/A Posted on - 11/18/2014

What is a primary key? What is a secondary key? What is a foreign key? What is the difference between each of them? Where are they used? What is a join? What are the different kinds of joins? Explain how they work? Can you differentiate between GROUP BY and ORDER BY?

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Answered By Shelock kate 0 points N/A #152199

Different types of SQL keys and their usage

A key is generally a single or combination of multiple fields 

in a database table. Their main use is to retrieve the 

required data from the table.The main types are-

1.Candidate key- A candidate is a combination of various columns

in a table. A number of candidate keys are allowed in a table.

2. Super key- A super key is a combination of various keys.

3. Primary key- It's used to distinguish between various columns

of a database table. A single key can be used a primary key.Null

values and duplication are not allowed in a primary key.

 

4. Alternate key -An alternate works as a primary key. It's a

candidate key but not a primary key.

5. Composite key- A combination of primary key and candidate key is 

termed as a composite key.

6. Unique key- It is a primary key which accepts only one NULL

value.

7. Foreign key- It is a primary key which contains NULL and 

duplicate values.

Hope it helps you. 

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• Posted on - 01/05/2015
• Question Category: Database