SDcols is an useful but tricky method in data.table. The statement of it is hard to remember, so I wrote down some examples, copying and pasting when I need them.

<p>If you are building your own modules or called codebase, it is common to split some huge functions as independent files. In other words, you may source functions in functions. In my case, I have a file named codebase, which includes many short functions. Also, I built some huge modules based on the codebase. However, when I keep improving the codebase, it is hard to update all huge modules to support the latest version of the codebase. So, the following code is really useful. Besides, it's a great way to prevent conflicts for the same name functions.</p>

Adding quotations or special characters to elements in vector, I think this a really useful feature. Especially, if you write SQL in R, this will be handy to you.

<p>data.table is a popular library in R, powerful, efficient. But the join/merge syntaxes in data.table are not so straightforward.<br />The common syntax as below which referred  〈JOINing data in R using data.table〉.</p>

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