title: Improving SQL query
body: <p>I'm trying to improve my SQL knowledge, and making sql-ex.ru excecises.</p>
  
  <p>And now I'm stuck with such problem.
  I have such task:</p>
  
  <p>*The database scheme consists of four tables:</p>
  
  <pre><code>Product(maker, model, type)
  PC(code, model, speed, ram, hd, cd, price)
  Laptop(code, model, speed, ram, hd, screen, price)
  Printer(code, model, color, type, price)
  </code></pre>
  
  <p>The table <strong>Product</strong> includes information about the maker, model number, and type ('PC', 'Laptop', or 'Printer'). It is assumed that model numbers in the Product table are unique for all the makers and product types.  </p>
  
  <p>Each <strong>PC</strong> uniquely specifying by a code in the table "PC" is characterized by model (foreign key referencing to Product table), speed (of the processor in MHz), total amount of RAM - ram (in Mb), hard disk drive capacity - hd (in Gb), CD ROM speed - cd (for example, '4x'), and the price.  </p>
  
  <p>The table <strong>Laptop</strong> is similar to that one of PCs except for the CD ROM speed, which is replaced by the screen size - screen (in inches). For each printer in the table "Printer" it is told whether the printer is color or not (color attribute is 'y' for color printers; otherwise it is 'n'), printer type (laser, jet, or matrix), and the price.*</p>
  
  <p><strong>Find the laptop models having speeds less than all PCs. 
  Result set: type, model, speed.</strong></p>
  
  <p>I'm typing such query:</p>
  
  <pre><code>SELECT 'Laptop', Laptop.model,
  Laptop.speed FROM Laptop WHERE
  Laptop.speed &lt; All (SELECT speed FROM
  PC)
  </code></pre>
  
  <p>It runs correctly, but the website tells that it's not totally correct for any database:</p>
  
  <p><em>Note that this decision will be valid only if the products with <code>laptop</code> type will be located in Laptop table. For our database this condition is satisfied, because there are only three types of products and, accordingly, three tables. Therefore, a violation of this condition may be associated only with the restructuring, which, however, should also be borne in mind when developing applications with built-in SQL queries.</em></p>
  
  <p>What does it mean? And how can I improve my query?
  Please, explain me!</p>