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How to use DISTINCT on just one column

Rudy Limeback EXPERT RESPONSE FROM: Rudy Limeback

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QUESTION POSED ON: 16 January 2008

Is there any way to use DISTINCT on just one column of a multi-column query? There have been numerous times I have wanted to do this, but have not found an easy way. In this instance I am selecting four columns and just want to distinct on the first column because the data of the four is different enough it returns duplicates of the first column.



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The answer to your question is yes, there is a way. Specify that column in the GROUP BY clause. Base every other expression in the SELECT on aggregate values only.

select column1
     , min(columns2)
     , count(columns3) + 3
     , sum(columns2*column4)
     , avg(columns4)/count(*)
  from yourtable
group
    by column1

The rule of thumb is: for every distinct combination of values you want (i.e. only one row per distinct such-and-such), put those columns into both the SELECT and the GROUP BY. Everything else in the SELECT must be based on aggregate expressions.



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