[datatable-help] using J() to select for a value that is in something other than the first key

[Matthew Dowle]

Hi. Try searching datatable-help for 'secondary'.

> In setkey it is possible to have multiple columns be a key. There is
> however an order to it. To select all the rows by a particular value in
> the
> first key is easy
> DT[J("a value from the first key")]
>
> It is even quite easy to select by an AND criteria that incorporates the
> second key
> DT[J("a value from the first key","a value from the second key")]
>
> But what happens if one wanted to select by only one criteria being a a
> value that must be present in the second key? I was hoping to get lucky by
> just leaving it blank. But it did not see to work.
>
> DT[J(,"a value from the second key")]
>
> I would love to know what you do.
>
>
> Farrel
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