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课程，但我很难理解其中一个问题，尽管我已经找到了答案。

架构如下：

/* 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：找出所有只供 24 岁以下的人食用，或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。基本上分为两部分：

24 岁以下的人吃的披萨
价格低于 10 美元的披萨

我什至理解答案，即（提供的所有比萨饼与价格大于等于 10 美元的比萨饼之间的差值）和（人们吃的所有比萨饼与大于等于 24 美元的人吃的所有比萨饼之间的差值）的并集岁）

(\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。按照我的说法，他们应该都给出相同的答案。

感谢所有回复。

Answer 1

您的第一个查询找到了

的比萨饼

花费不超过 10 美元任何地方，或者不被任何 24岁或以上的人食用，

而您上次查询找到的比萨饼

售价不到 10 美元某处，或 被一些 24岁以下的人吃。

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，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。

努力思考这个数据库查询

Struggling to wrap my head around this Database Query

sql

database

relational-algebra