Typical mistakes at the solution of SELECT statement exercises

Database "Computer firm"

Find out the models and prices for all the products (of any type) produced by maker B.

The products in the database can be of three types: PCs, laptops, and printers. The natural solution for this exercise would be union of three sets according to the type of products. This is how this exercise was solved by one of our participants:

Console

Execute

SELECT model, price 
FROM PC 
WHERE model = (SELECT model 
 FROM Product 
 WHERE maker = 'B' AND 
 type = 'PC'
 )
UNION
SELECT model, price 
FROM Laptop 
WHERE model = (SELECT model 
 FROM Product 
 WHERE maker = 'B' AND 
 type = 'Laptop'
 )
UNION
SELECT model, price 
FROM Printer 
WHERE model = (SELECT model 
 FROM Product 
 WHERE maker = 'B' AND 
 type = 'Printer'
 )

SELECT model, price FROM PC WHERE model = (SELECT model FROM Product WHERE maker = 'B' AND type = 'PC' ) UNION SELECT model, price FROM Laptop WHERE model = (SELECT model FROM Product WHERE maker = 'B' AND type = 'Laptop' ) UNION SELECT model, price FROM Printer WHERE model = (SELECT model FROM Product WHERE maker = 'B' AND type = 'Printer' )

    Scheme

Thereby within the main database this solution produces correct result, but within the verification database it outputs the following error:

That is to say, we cannot compare the individual value with the set, which is the case if the maker B manufactures more than one model of any type, which is exactly so for the verification database.