There isn't one perfect solution to SQL queries against complex systems.
A suduko has one solution.
A reasonably well-optimised SQL solution is what the good use of SQL tries to achieve. And it can be the difference between a total lock-up and a fast running of a script that keeps the rest of a complex system from falling over.
The number of solutions doesn't matter though. You can easily design a sudoku game that has multiple solutions, but it's still easier to verify a given solution than to solve it from scratch.
It's not even about whether or not the number of solutions is limited. A math problem can have unlimited amount of proofs (if we allow arbitrarily long proofs), but it's still easier to verify one than to come up with one.
Of course writing SQL isn't necessarily comparable to sudoku. But the difference, in the context of verifiability, is definitely not "SQL has no single solution."
