When you perform a systematic search, you need to consult more than one database. The number of databases depends on your search query.
Library subject guides
In the subject guides on the library website you can find tips of relevant subject-specific databases.
Sometimes it's relevant to search for ongoing or unpublished studies. In the medical research area you can find registers for clinical studies. Some examples are:
Find more examples of similar registry in the Cochrane handbook, part 6.2.3.