part3.txt

来自「ssd7的练习一」· 文本 代码 · 共 32 行

1.  In 1.2.2 Relational Operations, there was an example of the set difference operation: the result of r - s was provided to you. Provide the result of the set difference operation s - r. 
		
Answer:				
					Table r - s 
				D		E		F
			=================================================	
				b		g		a

2. In the class notes, the following UNION example was discussed. Give a prosaic description of the results that will be retrieved by replacing UNION with EXCEPT in the following query: 
               ( SELECT   MemNo
                  FROM     MEMBER, BOOK
                  WHERE    MemNo = BorrowerMemNo AND
                  CallNumber = 'QA76.9.D26C66'
                )
                UNION
                ( SELECT   MemNo
                  FROM     MEMBER, BOOK
                  WHERE    MemNo = BorrowerMemNo AND
                  CallNumber = 'QA76.9.D7E53'
                );

 

Answer:
Following will be retrieved by replacing UNION with EXCEPT:
A list of member numbers of the members that only borrow the book that has the call number QA76.9.D26C66 but not borrowing the book that has the call number QA76.9.D7E53.