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Typical errors at solution of exercises. Exercise 2

S. Moiseenko

Find printer makers. Result set: maker.

Typical mistake when solving exercise #2 is the query:

select distinct maker
   from product, printer
   where product.model=printer.model

The Product and Printer tables are in one-to-many relationship. In particular, it means that Printer table may not involve all the printer models available in Product table. As a result, the above query will return only those models, which are available (!) in Printer table.

As we need to find all manufacturers of printers it is necessary to use only Product table, filtering the printers by using [type] column where product type is pointed.

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