SD in its second usage, grouping scenarios (i.e., when by = or keyby = is specified), is slightly different, conceptually (though at core it’s the same, since, after all, a non-grouped operation is an edge case of grouping with just one group). SDcols is specified) as this version is much more straightforward to understand, we’ll cover that first below.
SD is itself a data.table (with the caveat that it does not allow assignment with :=). SD is in its most basic guise a reflexive reference to the data.table itself – as we’ll see in examples below, this is particularly helpful for chaining together “queries” (extractions/subsets/etc using [). It can be understood to stand for Subset, Selfsame, or Self-reference of the Data.
SD is just shorthand for capturing a variable that comes up frequently in the context of data analysis.
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