I find it odd that SQL is hardly ever mentioned in this type of lists. Sooner or later you will have to deal with databases. And yes, i consider SQL a full programming language. Many modern databases have (turing complete) server side processing with at least one mode using the SQL language as a basis.
Of course, PostgreSQL takes the cake, again, because it also supports Perl, Python, TCL and whatever you like. Which allows you to support weird stuff like
SELECT title, url FROM perlmonks WHERE supersearch like '%learning%perl%';
But you'd still need to know SQL to get going.
perl -e 'use Crypt::Digest::SHA256 qw[sha256_hex]; print substr(sha256_hex("the Answer To Life, The Universe And Everything"), 6, 2), "\n";'
