Exercise #15 (computer firm) | Interactive tutorial on SQL

	Common mistakes in solving SELECT statement exercises Computer Firm Database Exercise #15
	Find the hard drive sizes that are equal among two or more PCs. Result set: hd. A superficial knowledge of data structure is the reason of incorrect solution: Solution 1.11.1 Console Execute SELECT DISTINCT t.hd FROM PC t WHERE EXISTS (SELECT * FROM PC WHERE pc.hd = t.hd AND pc.model <> t.model ); SELECT DISTINCT t.hd FROM PC t WHERE EXISTS (SELECT * FROM PC WHERE pc.hd = t.hd AND pc.model <> t.model ); Scheme This query extracts those PCs for which an another model of PC with same size of hard drive exists. The mistake is in the intuitive notion of the model uniqueness in the PC table. However, as was mentioned early, a model numbers are unique only in Product table. But here a numbers can be not unique. So, the query is wrong because the same models with same hard drive size are not taken into account. T&S To solve the problem on SQL-EX.RU mistakes PC EXISTS exercise 15

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