嗨，有人可以请证实我的工作。我不确定我是否正确地做了这些，并非常感谢任何帮助。我不允许使用领结操作员。谢谢。
问题：
Books (ISBN, Title, Authors, Publisher, Ed, Year, Genre)
Patron (MemberNumber, FirstName, LastName, AddressLn1, AddressLn2, City, State, Zipcode)
Loan (MemberNumber,ISBN,DateLoaned,DateDue, DateReturned)
Business Logic
• You may assume that the library only has one copy of each book.
• Each book may have many authors. If a particular book has multiple authors, they are listed as a comma separated string. You may assume that the same author always uses the same exact name and no two authors will have the same name.
• Year is stored as an integer.
• DateLoaned, DateDue, and DateReturned are stored as a date.
• When a book is initially lent out, DateReturned is set to be NULL, upon its return, the value is updated.
1.1. Find all books that were loaned out after 12/22/2012. Show the ISBN, Title, and DateDue.
1.2. Find all library patrons who have borrowed a book titled "Database Systems". Show their FirstName, LastName, and DateLoaned.
1.3. Find all books that were ever loaned out. Display the ISBN.
1.4. Find all books returned before 12/22/2012. Display the ISBN.
1.5. Find all books returned on or after 12/22/2012. Display the ISBN.
1.6. Find all books returned either (before 12/22/2012) or (on or after 12/22/2012) Display the ISBN.
1.7. In 1 sentence explain the difference between 1.3 and 1.6.
1.8. Find all patrons who have never borrowed a book.
1.9. Find all books with Genre "Mystery" that have NEVER been loaned out.
1.10. Create a new attribute ImportantDates. A date is important if it is in the Loan relation either as a DateLoaned or a DateDue. Display ImportantDates.
1.11. Find all library patrons who have borrowed a book with an author "James Stewart". You may use the expression LIKE "%James Stewart%" in your Relational Algebra.
1.12. Find all library patrons who have never borrowed a book with an author "James Stewart". You may use the expression LIKE "%James Stewart%" in your Relational Algebra.
1.13. Find all library patrons who have only borrowed a book with an author "James Stewart". If they have ever borrowed a book without the author "James Stewart" they should be excluded. You may use the expression LIKE "%James Stewart%" in your Relational Algebra.
答案：
1.1) πISBN,TITLE,DATEDUE(σDATELOANED > 12222012 AND BOOKS.ISBN = LOAN.ISBN(LOAN X BOOKS)
1.2) πFIRSTNAME,LASTNAME,DATELOANED(σTITLE = "DATABASE SYSTEMS" AND BOOKS.ISBN = LOAN.ISBN AND PATRON.MEMBERNUMBER = LOAN.MEMBERNUMBER(BOOKS X PATRON X LOAN))
1.3) πISBN(σDATELOANED <> "NULL" AND BOOK.ISBN = LOAN.ISBN(LOAN X BOOKS))
1.4) πISBN(σDATELOANED < 12222012 AND BOOK.ISBN = LOAN.ISBN(LOAN X BOOKS))
1.5) πISBN(σDATELOANED >= 12222012 AND BOOK.ISBN = LOAN.ISBN(LOAN X BOOKS))
1.6) πISBN(σDATELOANED >= 12222012 OR DATELOANED <12222012 AND BOOK.ISBN = LOAN.ISBN(LOAN X BOOKS))
1.7) 1.3 AND 1.6 are the same as they both find books that have been loaned.
1.8) σDATELOANED = "NULL" AND PATRON.MEMBERNUMBER = LOAN.MEMBERNUMBER(LOAN X PATRON)
1.9) σGENRE = "MYSTERY" AND BOOKS.ISBN = LOAN.ISBN AND DATELOANED = "NULL"
1.10) LOAN(DATELOANED,DATEDUE) -> IMPORTANTDATE
能不能给我一个1.11,1.12或1.13的例子，因为我不知道如何使用LIKE表达式。
答案 0 :(得分：1)
教授告诉你该怎么做：
You may use the expression LIKE "%James Stewart%" in your Relational Algebra.
自从我不得不使用关系代数已经有一年了，但是无论你的前提条件是什么（这都是你的任务），接下来就是这一行：
LIKE %James Stewart%
SQL语句看起来像这样：
Select * from patrons p where Books.author LIKE %James Stewart%
你会发现在你的学习中，关系代数不处理SQL的功能，它只是看待事物的纯粹数学方面。