We can use special symbols, such as `.SD`, `.EACHI`, `.N`, `.I`, and `.BY` in `data.table`, to enhance the functionality. We already saw some examples of `.N`, which represents the number of rows or the last row.

The `.SD` symbol holds all columns except the columns in `by`, and can be used only in the `j` evaluation part of `data.table`. The `.SDcols` symbol is used along with `.SD` and has columns to be included or excluded in the `j` part of `data.table`.

The `.EACHI` symbol is used in the `by` grouping to group each subset of the groups in `i`. This needs a key to be defined. If there is no key, R throws an error.

In the following example, we calculate the maximum salary in each department. If we omit `.SDcols="Salary"`, then R will try to find the max for all ...