Could you give us a situation where both DISTINCT and GROUP BY are required?

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Imagine a village of many families, each consisting of family members. Someone with no relatives whatsoever is a family of one. Now let us take a survey of the population, and ask, "What are the different family sizes in this village?"

create table families
( familyid tinyint 
, person varchar(9)
insert into families values
,(1,'curly joe')
select distinct count(*) as familysize
  from families
group by familyid

The result set is 6, 1, 3. Without DISTINCT, it would be 6, 1, 3, 3. Add a few hundred more families of size 3 and you will notice the difference.

This was first published in October 2005

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