A Systematic Review is a very special beast. Originating in Medicine as an aspect of "evidence based medicine", the systematic review has now been adopted in the Social Sciences. The key aspect is of the systematic review is replicability.
Rather than relying on individual judgment to select items from the literature, the systematic review aims to produce a list of sources which can be replicated by anybody using the same search strategies on the same databases.
For this reason, every aspect of the search, and every decision made about sources used or search terms chosen, must be recorded. Frequently a team of researchers is involved, and, where results are hand picked, this is done separately by two or three individuals, working simultaneously on the same material.
It is strongly advised that you work with a librarian in planning your searches and identifying your databases.