Struggling to wrap your head around this database query

Question

I am completing a course at Stanford Relational Algebra, and it is difficult for me to understand one of the questions, although I received an answer.

Here is the diagram:

/* 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);

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I'm struggling to wrap my head around question 6. The question is: Find all pizzas that only people under the age of 24 eat, or that cost less than $ 10 in everything that is served to them.

Now I understand the request. It is mainly divided into two parts:

• Pizza eaten by people under 24 years old
• Pizza costing less than $ 10

I even understand the answer, i.e. Union (the difference between all pizzas and pizzas that cost more than 10%) and (the difference between all pizzas consumed by people and those who ate people over 24 years of age)

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

, , .. ( 10 $) (, 24 ).

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

, . , .

.

Answer 1

,

, $10 , 24 ,

,

$10 -, 24.

, 10 ​​ 10 24 .