我是斯坦福大学关系代数课程的学习者,但我在理解其中一个问题时遇到了困难,尽管我已经得到了答案。
以下是模式:
我很难理解第6个问题。问题的内容是:“找到所有只被24岁以下人吃或在它们被提供的所有地方都少于10美元的披萨。”现在,我理解这个查询。它基本上分为两部分:
1.由24岁以下人吃的披萨 2.价格低于10美元的披萨
我甚至理解答案,即(所有提供的披萨与价格大于等于10美元的披萨之间的差异)和(所有人吃的披萨与年龄大于等于24岁的人所吃的披萨之间的差异)的联合。
为什么第二个查询的答案与第一个不同。在我看来,它们应该都给出相同的答案。
感谢所有回复。
以下是模式:
/* Delete the tables if they already exist */
drop table if exists Person;
drop table if exists Frequents;
drop table if exists Eats;
drop table if exists Serves;
/* Create the schema for our tables */
create table Person(name text, age int, gender text);
create table Frequents(name text, pizzeria text);
create table Eats(name text, pizza text);
create table Serves(pizzeria text, pizza text, price decimal);
/* Populate the tables with our data */
insert into Person values('Amy', 16, 'female');
insert into Person values('Ben', 21, 'male');
insert into Person values('Cal', 33, 'male');
insert into Person values('Dan', 13, 'male');
insert into Person values('Eli', 45, 'male');
insert into Person values('Fay', 21, 'female');
insert into Person values('Gus', 24, 'male');
insert into Person values('Hil', 30, 'female');
insert into Person values('Ian', 18, 'male');
insert into Frequents values('Amy', 'Pizza Hut');
insert into Frequents values('Ben', 'Pizza Hut');
insert into Frequents values('Ben', 'Chicago Pizza');
insert into Frequents values('Cal', 'Straw Hat');
insert into Frequents values('Cal', 'New York Pizza');
insert into Frequents values('Dan', 'Straw Hat');
insert into Frequents values('Dan', 'New York Pizza');
insert into Frequents values('Eli', 'Straw Hat');
insert into Frequents values('Eli', 'Chicago Pizza');
insert into Frequents values('Fay', 'Dominos');
insert into Frequents values('Fay', 'Little Caesars');
insert into Frequents values('Gus', 'Chicago Pizza');
insert into Frequents values('Gus', 'Pizza Hut');
insert into Frequents values('Hil', 'Dominos');
insert into Frequents values('Hil', 'Straw Hat');
insert into Frequents values('Hil', 'Pizza Hut');
insert into Frequents values('Ian', 'New York Pizza');
insert into Frequents values('Ian', 'Straw Hat');
insert into Frequents values('Ian', 'Dominos');
insert into Eats values('Amy', 'pepperoni');
insert into Eats values('Amy', 'mushroom');
insert into Eats values('Ben', 'pepperoni');
insert into Eats values('Ben', 'cheese');
insert into Eats values('Cal', 'supreme');
insert into Eats values('Dan', 'pepperoni');
insert into Eats values('Dan', 'cheese');
insert into Eats values('Dan', 'sausage');
insert into Eats values('Dan', 'supreme');
insert into Eats values('Dan', 'mushroom');
insert into Eats values('Eli', 'supreme');
insert into Eats values('Eli', 'cheese');
insert into Eats values('Fay', 'mushroom');
insert into Eats values('Gus', 'mushroom');
insert into Eats values('Gus', 'supreme');
insert into Eats values('Gus', 'cheese');
insert into Eats values('Hil', 'supreme');
insert into Eats values('Hil', 'cheese');
insert into Eats values('Ian', 'supreme');
insert into Eats values('Ian', 'pepperoni');
insert into Serves values('Pizza Hut', 'pepperoni', 12);
insert into Serves values('Pizza Hut', 'sausage', 12);
insert into Serves values('Pizza Hut', 'cheese', 9);
insert into Serves values('Pizza Hut', 'supreme', 12);
insert into Serves values('Little Caesars', 'pepperoni', 9.75);
insert into Serves values('Little Caesars', 'sausage', 9.5);
insert into Serves values('Little Caesars', 'cheese', 7);
insert into Serves values('Little Caesars', 'mushroom', 9.25);
insert into Serves values('Dominos', 'cheese', 9.75);
insert into Serves values('Dominos', 'mushroom', 11);
insert into Serves values('Straw Hat', 'pepperoni', 8);
insert into Serves values('Straw Hat', 'cheese', 9.25);
insert into Serves values('Straw Hat', 'sausage', 9.75);
insert into Serves values('New York Pizza', 'pepperoni', 8);
insert into Serves values('New York Pizza', 'cheese', 7);
insert into Serves values('New York Pizza', 'supreme', 8.5);
insert into Serves values('Chicago Pizza', 'cheese', 7.75);
insert into Serves values('Chicago Pizza', 'supreme', 8.5);
1.由24岁以下人吃的披萨 2.价格低于10美元的披萨
我甚至理解答案,即(所有提供的披萨与价格大于等于10美元的披萨之间的差异)和(所有人吃的披萨与年龄大于等于24岁的人所吃的披萨之间的差异)的联合。
(\project_{pizza} (Serves) \diff \project_{pizza} (\select_{price>='10'} Serves))
\union
(\project_{pizza} (Eats) \diff \project_{pizza} (\select_{age>='24'} Person \join Eats))
然而,我不明白为什么相反的查询没有给出相同的答案,即(pizzas价格低于10美元)和(被24岁以下人吃的披萨)的联合。
(\project_{pizza} (\select_{price<'10'} Serves))
\union
(\project_{pizza} (\select_{age<'24'} Person \join Eats))
为什么第二个查询的答案与第一个不同。在我看来,它们应该都给出相同的答案。
感谢所有回复。