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

[Farrel Buchinsky]

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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