I'm not legally qualified, so this is just my view.

It depends on the contract between the client and the consultant, which may be as simple as a letter. If the consultant was only engaged to design and set up the database application, and the client entered all the data, the client should be entitled to recover all the data from the system. As always with contracts and the law, enforcement of the right is the problem.

If the client provided the data in some other form to the consultant, who incorporated it into the database, the consultant may be able to argue that the client already has the data, and is only entitled to the outputs that were called for by the contract.

One possible lever for the client is that if the database system is no longer to be used, the consultant has no right to retain any of the data.

I'm sure others can produce better advice.

Phil