the first thing you must (un)learn is the name of the microsoft product
microsoft has a product called
sql server (actually, i think sybase does too, although they do not, of course, market it very heavily by that name; microsoft actually licensed it from sybase, but that's another story...)
sql always refers to the
language -- structured query language (and may i point out that it's neither structured not just queries)
the differences between mysql and microsoft sql server (also called mssql) are too numerous to list
in a nutshell, mysql and sql server are both database systems, and both use a variant of standard sql, but that's pretty much all you can say about their similarities
interchangeable as regards development? yes, to the extent that they both use sql, so you have a large body of syntax which is common to both systems
it's the differences in syntax you have to watch out for, but once you know them, it's not that hard
for example, any date calculations you want to do, mysql is a lot closer to standard sql syntax than sql server, but that's because sql server was created before the date standards were completed
yes you can develop an asp application with mysql as your database, just be prepared to tweak the sql when you change over (and make whatever changes are necessary to the connection code)