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Exercise #17 (tips and solutions)

So, get rid of the Cartesian product. To do this, we remove the PC table from FROM clause, and join Product and Laptop tables on ‘model’ column:

Console
Execute
  1. SELECT DISTINCT type, Laptop.model, speed
  2. FROM Laptop, Product
  3. WHERE Product.model = Laptop.model AND
  4.       Laptop.speed < (SELECT MIN(speed) FROM PC);

Condition p.type = 'laptop' is unnecessary, because the internal joining will contain only model of this type. It turns out that join is no needed also, because Product table is used only for product type output in result set. But the product type is known in advance that a laptop computer, therefore we can simply use the expression (a constant) to specify the type, removing the join:

Console
Execute
  1. SELECT DISTINCT 'Laptop', model, speed
  2. FROM Laptop
  3. WHERE speed < (SELECT MIN(speed) FROM PC);

Note that this decision will be valid only if the products with ‘laptop’ 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(Structured Query Language) is a database computer language designed for the retrieval and management of data in relational database management systems (RDBMS), database schema creation and modification, and database object access control management.SQL queries.

To return to discussion of exercise #17

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