arowa704

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