There are varied opinions about this topic. Several Sources of Truth and Righteousness advise against "sequel"[1][2], However it seems that in a certain building in WA "sequel" is preferred [3].
Redmond also colloquially refers to the product Microsoft SQL Server as just SQL (pronounced "sequel"), as in, "We're trying sell more SQL licenses," or, "Is the backing DB for your SAP installation Oracle or SQL?"
This has led to some confusion.
Additionally, as I pointed out in a sibling comment up the chain somewhere, SQL is an abbreviation of the original acronym, SEQUEL, for Structured English QUEry Language. It seems with this provenance that the "sequel" pronunciation is more historically correct.
[1] https://dev.mysql.com/doc/refman/5.7/en/what-is-mysql.html
[2] http://www.postgresql.org/about/press/faq/
[3] https://social.msdn.microsoft.com/Forums/sqlserver/en-US/fd6...