I've placed this answer because Erwin Smout posted a answer that was so wrong it highlighted that there is probably a need to specifically guard against it.
Erwin suggested:
"Why should every SELECT always include DISTINCT ?"
A more appropriate question would be: If someone were to make the claim that: "every SELECT always include DISTINCT"; how would you comment on the claim?
If a candidate is unable to shoot the claim down in flames they either:
- Don't understand the problem with the claim.
- Lack in critical thinking skills.
- Lack in ability to communicate technical issues.
For the record
- Suppose your query is correct, and does not return any duplicates, then including DISTINCT simply forces the RDBMS to check your result (zero benefit, and a lot of additional processing).
- Suppose your query is incorrect, and does return duplicates, then including DISTINCT simply hides the problem (again with additional processing). It would be better to spot the problem and fix your query... it'll run faster that way.
