sql - 努力思考这个数据库查询

Question

我正在完成斯坦福大学的关系代数类(class)，但我很难理解其中一个问题，尽管我已经找到了答案。

这是架构:

/* 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);

我正在努力思考问题 6。问题内容如下:找到所有只供 24 岁以下的人食用，或者在任何地方提供的价格都低于 10 美元的比萨饼。

现在，我理解了查询。它基本上分为两部分:

1. 24 岁以下的人吃的披萨
2. 价格低于 10 美元的披萨

我什至理解答案，即(所有供应的比萨饼与价格大于等于 10 美元的比萨饼之间的差异)和(人们吃的所有比萨饼与大于等于 24 年的人吃的所有比萨饼之间的差异)的联合年龄)

(\project_{pizza} (Serves) \diff \project_{pizza} (\select_{price>='10'} Serves))
\union
(\project_{pizza} (Eats) \diff \project_{pizza} (\select_{age>='24'} Person \join Eats))

但是，我不明白为什么相反的查询没有给出相同的答案，即 Union of (pizzas costing less than 10$) and (pizzas eated by people less than age of 24)。

(\project_{pizza} (\select_{price<'10'} Serves))
\union
(\project_{pizza} (\select_{age<'24'} Person \join Eats))

为什么第二个查询给出的答案与第一个不同。在我看来，他们应该给出相同的答案。

感谢所有回复。

Answer 1

您的第一个查询找到比萨饼

在任何地方的花费都不会超过 10 美元，或未被任何 24 岁或以上的人食用，

当你的最后一个查询找到比萨饼时

在某处以低于 10 美元的价格出售，或者被 24 岁以下的一些人吃掉。

对于在某些地方售价低于 10 美元而在其他地方售价超过 10 美元的比萨饼，或者由一些 24 岁以下的人和一些 24 岁以上的人吃的披萨，这些查询将给出不同的结果。