There isn't a standard. Of course , there are database server platforms that have better employment opportunities, but this can change. You may find over your career you become proficient in more than one database server platforms- for example - Primary skills :SQL Server, Secondary skills = MySQL. Here are some other comments: http://www.sqlserver-dba.com/2013/01/sql-server-i-want-to-become-a-dba-what-should-i-first-learn-.html