Part of being an ethical student is using APA citations to properly credit the ideas and words you use in your assignments. The link below provides more information on academic integrity at Concordia University and on plagiarism.
These provide visual examples of reference list citations.
Q: If I used a database and there is no DOI, why can't I just include the database information all the time?
A: Try to get your readers to the material in a universally accessible, simple, and direct way. Many databases are subscription based or university specific and thus not accessible to everyone. Their content can also change over time. Although publisher homepages may also change (which is why DOIs are preferred), they are accessible to everyone and easy to find online.