Can there be any open/free database service with web-based connects for testing queries?
Added: It is going to be great for this case If I am troubling with complex queries, then produce a sample table via web interface and request it on stackoverflow using the 'sample table URL'. Anybody can accessibility URL and test their queries on site. (Yes, queries are running on 'real' database system) As well as the query results could be monitored, only then do we can make 'ranking' for this :)
So far as the net-based connects and short Web addresses go, I do not think you will have much luck.
To handle your computer data you need to stay with what's provided (usually phpMyAdmin or similar) and there's no short-Hyperlink to query mapping. Another caveat of these product is that (with no appropriate user permissions) one user could easily destroy all of your test data -- and don't forget that (relational) database versioning is a lot more costly than plain text versioning, so that's virtually unthinkable.
For non-RDBMS, I'm able to think about try.mongodb.org -- however it is affected with exactly the same problems.
Almost didn't remember, the Stack Exchange Data Explorer, allows you practice T-SQL queries (with permalinks).
PS: Like a personal side-note, I believe it is a awesome idea and that i would like to see something of that nature implemented, possibly even mashed-track of SchemaBank or similar - that might be just awesome.
Challenge with an internet based query analyser is the fact that you'd have to allow it to 'tunnel' onto your box to operate the queries as well as for many creating a development/test box open to the web isn't a possibility.
For any non internet based tool you could think about LinqPad http://www.linqpad.net/ - it will Linq &lifier Sql along with other stuff too - very handy tool indeed
You will never test a question without proper underlying dbms, schemas (or databases), tables, constraints, saved methods, and permissions, which are usually highly application specific. (That's, not readily multiple-use among multiple customers.)
Rather, the database world is growing up into database management systems that you could freely download and install in your area. You'll be able to build and populate your personal tables, and test out your queries any way you like.
Many of these include both an order line interface plus some type of graphical interface. It isn't obvious in my experience exactly what a web-based interface gives you that does not already appear in one form or any other.
I believe that, to complete what you would like, would require commercial licenses for Oracle, DB2, SQL Server, and Sybase. This is a pretty high barrier to entry for any free site.