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“SQL-99 Complete, Really”, by Peter Guzman & Trudy Pelzer
This is a 3 pages article suggested by the instructor but I could not find the link. In google scholar search result, this reference information is there but I could not find the related pages.
I do not have any university library collection network access right now. If you have the access, please search and share this reading as .pdf file for treehouse community.
 Peter Gulutzan y Trudy Pelzer, SQL-99 Complete Really, Primera Edición, R & B Books, Lawrence, Kansas, 1999, pp. 627-630.