"Safely" allow users to search with SQL

Question

For example I've often wanted to search stackoverflow with

SELECT whatever FROM questions WHERE
   views * N + votes * M > answers AND NOT(answered) ORDER BY views;

or something like that.

Is there any reasonable way to allow users to use SQL as a search/filter language?

I see a few problems with it:

• Accessing/changing stuff (a carefully setup user account should fix that)
• SQL injection (given the previous the worst they should be able to do is get back junk and crash there session).
• DOS attacks with pathological queries
• What indexes do you give them?

Edit: I'd like to allow joins and what not as well.

Answer 1

Accessing/changing stuff
No problem, just run the query with a crippled user, with permissions only to select

SQL injection
Just sanitize the query

DOS attacks
Time-out the query and throttle the access by IP. I guess you can also throttle the CPU usage in some servers

Answer 2

If you do SQLEncode your users' input (and make sure to remove all ; as well!), I see no huge safety flaw (other than that we're still handing nukes out to psychos...) in having three input boxes - one for table, one for columns and one for conditions. They won't be able to have strings in their conditions, but queries like your example should work. You will do the actual pasting together of the SQL statement, so you'll be in control of what is actually executed. If your setup is good enough you'll be safe.

BUT, I wouldn't for my life let my user enter SQL like that. If you want to really customize search options, give either a bunch of flags for the search field, or a bunch of form elements that can be combined at will.

Another option is to invent some kind of "markup language", sort of like Markdown (the framework SO uses for formatting all these questions and answers...), that you can translate to SQL. Then you can make sure that only "harmless" selects are performed, and you can protect user data etc.

In fact, if you ever implement this, you should see if you could run the commands from a separate account on the SQL server, which only has access to the very basic needs, and obviously only read access.