For all I know:

'work in a hospital' has got only one meaning. It means that you work inside the building.

When it comes to 'work at a hospital', it can have two meanings. First, it means exactly the same as 'work in a hospital'. Apart from this one, it may also mean that you work next to the hospital building, somewhere in the area surrounding the hospital building. For example: 'I work in the kiosk at the hospital building'.

@Ha:

Here are some ideas to share with you:

1. Many books say that either "in" or "at" is correct in your sentence.

2. Some books see a difference:

a. Some people (especially Americans?) feel this way:

i. "at" refers to a point on the map:  I work AT a hospital in New York; I work AT a hospital near the railroad station; etc.

ii. "in" refers to the actual place in which you work: I work in a hospital, not in a bank.

according to my British Council teacher,

both are ok, but if you want to say, i work at hospital as a doctor or something, then "work at hospital" is better.
becuz, if you say 'work in a hopital' could be as a hoover fixer for temporarily.

Both seem fine to me. I can't see a difference. '...in a hospital' is more common.

well,