Ready web query interface to SQL databases [closed]
The StackExchange Data Explorer has this functionality (example) and it is open source, so you may be able to just install and make use of it, or even extract the relevant code.
Perhaps something like the SQL Fiddle?
Obviously, this is mainly for demonstration purposes - it won't help you if you want to install it on top of your own database.
Re:dash (Redash) is a promising new open-source player on the market. From their own website:
re:dash is our take on freeing the data within our company in a way that will better fit our culture and usage patterns.
Prior to re:dash, we tried to use traditional BI suites and discovered a set of bloated, technically challenged and slow tools/flows. What we were looking for was a more hacker'ish way to look at data, so we built one.