Can't the Home Office do basic SQL and basic maths?

You know how we're constantly being told about the wonders of Information Technology by the Government? How they're going to create these wonderful databases that can help us with identity, benefits, child protection etc through marvelous and automagical data-sharing across department?

Putting aside the Orwellian overtones of such things I've always though it sounded like a pipe dream because they're too incompetent, and having just read Hansard I think I might be right.

When Alan Johnson, the new Home Secretary, was asked by Chris Grayling, how many people not convicted of a crime have their fingerprints recorded on the National Fingerprint Database the response was as follows.

The National Fingerprint Database does not hold criminal conviction data; it stores biometric data and basic identity details which can be used to align identity with records on the Police National Computer (PNC). The PNC is an operational tool and not designed to produce the information requested. To obtain the information would incur disproportionate cost.

Hmmm, so we have two databases, one storing fingerprints (NFD) and another that contains identity details on the Police National Computer (PNC). Apparently the fingerprints can be "aligned" with the PNC data.

This suggests that there is some sort of referential key in the PNC that can map to records in the NFD. If the PNC holds data on convicted criminal then all you actually need to do is take the total number on the PNC and subtract it from the total number on the NFD, and hey presto, you have the number of people on the NFD who have no criminal conviction on the PNC.

Can they seriously be saying that it will cost more than £700 to do a record count on two database and then subtract one value from the other?