求问数据库基本概念的题，题目如图

Question

答案没看懂，可否来个大神解释下，感激不尽
a. t1 ←Πisbn(σpublisher=“McGraw−Hill′′ (books))
Πname ((member ⋈ borrowed) ⋈ t1))
b. t1 ←Πisbn(σpublisher=“McGraw−Hill′′ (books))
Πname,i sbn(member ⋈ borrowed) ÷ t1
c. t1 ←member⋈ borrowed⋈ (σpublisher=“McGraw−Hill′′ (books))
Πname (σcountisbn > 5 (( memb_no Gcount-distinct(isbn) as countisbn(t1))))
d. t1 ←member⋈borrowed⋈books
⋈publisher,name (σcountisbn > 5 (( publisher, memb_no Gcount-distinct(isbn) as countisbn(t1)))

Answer 1

主要是一些符号，你可以看你书上的例题照着写就可以了。比如σ，是表示取它的字段。