data.table Joins and merges
Introduction
A join combines two tables containing related columns. The term covers a wide range of operations, essentially everything except appending the two tables. "Merge" is a synonym. Type
?`[.data.table` for the official docs.
Syntax
- x[i, on, j]
# join: data.table x & data.table or list i - x[!i, on, j]
# anti-join
Remarks
Working with keyed tables
If x & i have a key or x is keyed to match i's first few columns, then the on can be skipped like x[i].
Disambiguating column names in common
In j of x[i, on, j], columns of i can be referred with i.* prefixes.
Grouping on subsets
In j of x[i, on, j, by=.EACHI], j is computed for each row of i.
This is the only value of by worth using. For any other value, columns of i are not available.
