[Primary Key and Unique confusion]

hi,

AFter reading this paragraph from a book, I got confused:

 

\"While PRIMARY KEY forbids NULL\'s in the attributes of the key, UNIQUE permits them. Moreover, the rule that two tuples may not agree in all of a set of attributes declared UNIQUEmay be violated if one or more of the components involved have NULL as a value. In fact, it is even permitted for both tuples to have NULL in corresponding attributes of the UNIQUE key.\"

 

What I got from this:

1) PRIMARY KEY cannot have NULL

2) UNIQUE can have NULL

 

From Moreover on... is where I got confused.

1) 2 tuples can have NULL in an UNIQUE attribute, which is legal....

 

Another Question

A Primary Key is Unique indirectly...?

But I\'m assuming those two keywords can never be used together.

 

Correct me If i\'m wrong. Thanks in advance