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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