title: Relational algebra ONLY & EVERYWHERE
body: <p>Here is a simple database </p>
  
  <pre><code>/* 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);
  </code></pre>
  
  <p>The question is 'Find all pizzas that are eaten only by people younger than 24, or that cost less than $10 everywhere they're served'.The problem comes here :</p>
  
  <pre><code>\project_{pizza} ( Eats \join \select_{age &lt; 24 } Person)
  </code></pre>
  
  <p>I write this way but there's one thing lacking.It says only by people younger than 24.Similarly for the second 'everywhere served' problem still exists.How can I limit that??</p>