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SQL Problems and solutions S. I. Moiseenko

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Common mistakes in solving SELECT statement exercises

Computer Firm Database

Exercise 3 (rating)

For Product table, receive result set in the form of a table with columns: maker, pc, laptop, and printer.For each maker, this table must include "yes" if a maker has products of corresponding type or "no" otherwise.In the first case (yes), specify in brackets (without spaces) the quantity of available distinct models of corresponding type (i.e. being in PC, Laptop, and Printer tables).

If the manufacturer has models of some type in the Product table, but none of them is available, then according to the formulation the result should be


yes(0)

Instead of


no

Many find a "mistake" in the test solution. Claims are reduced to the following query:

Console

Execute

SELECT COUNT(*)
   FROM Product
   WHERE Maker = 'E' AND type='PC';

which gives 3 models of PC for manufacturer Е while "right answer" gives only one model of a computer for this manufacturer. We need return to the formulation that tells us:

“… specify in brackets (without spaces) the quantity of available distinct models of corresponding type (i.e. being in PC, Laptop, and Printer tables).”

In our case, this means that we need specify in brackets a quantity of distinct models of the personal computer of manufacturer Е in PC table. In terms of 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 it means:

Console

Execute

SELECT COUNT(DISTINCT pc.model)
   FROM Product pr
   JOIN PC ON pr.model=pc.model
   WHERE Maker = 'E';

i.e.


1

To solve the problem on SQL-EX.RU

mistakes Product Computer firm exercise 3 (rating)

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SQL Problems and solutions

S. I. Moiseenko

Common mistakes in solving SELECT statement exercises

Computer Firm Database

Exercise 3 (rating)