NULL Types

 2 NULL types:

A-Values: The value exists but we don’t know what it is.

I-Values: The attribute is irrelevant.

They both have different meanings, but they are represented by the same NULL.

You should remember that when:

• Design a table.
• Design column values.
• Query table.

ACID

ACID is a set of properties that guarantee database transactions are processed reliably:

• Atomicity, 
• Consistency, 
• Isolation,
• Durability.

http://en.wikipedia.org/wiki/ACID

But, CAP theorem states that it is impossible for a distributed computer system to simultaneously provide all three of the following guarantees:

• Consistency,
• Availability,
• Partition tolerance.