Do you mean like providing a TextBox the user can enter "WHERE Salary>=2000"?

I'd rather first check, if the way i'd usually do something like this would be feasable (if i ever would get such an order):
Think Mac-Finder-App --> kinda like an Explorer with Miller-Columns

First column: available tables to choose from (if it's always the same table, then you don't need that one)
Next column: available columns
after selecting a column (move it from left ListBox to right Listbox?) offer a Dialogue/TextBox to enter the value, via Dropdown the operator ("=", "<", "LIKE" etc.), and optional the boolean operator for the next one (AND/OR/NOT)

It's just a concept-idea of mine. No Idea if it's practicable.
OTOH, the DMS at the company i work for provides exactly such a dialogue to the user if he's searching a document in the archive

Additionally, you wouldn't really need to think about injection (i take it you'll use parameters anyway)