Studying architecture so you can do interior design seems like you would be studying a lot of material (architecture) that you have little interest in for a little study in interiors. Several large schools have it all: architecture, landscape architecture, interior design, construction management, real estate development, civil engineering. I would try to go to one of those schools and talk with the professors and students in each area to find out what you like and what jobs were available to recent graduates. If it is still interior design after your first year you are all set. If you want to change majors you are also well positioned.  

you can be an interior designer with a high school diploma.   you could also do it without a diploma 

Yes, we do it in-house (residential practice in architecture/interiors/landscape 20+ people firm).  A lot has to do with ability to understand style/color/texture at a level above the architecture, but the spatial understanding of an architecture education can be very useful.

I'd recommend looking at programs like SAIC's Interior Architecture degree. It may be a good program if you are interested in the interiors as opposed to the more traditional approach to architecture. The program is expensive so be warned. Some people on here may say that Interior Architecture is just another title for Interior Design, which in most cases I sort of agree, however in the right program, with the right professors, and  motivation, Interior Architecture could be a unique niche that is separate from interior design. 

Some states do have licensure for Interior designers.  So, you should probably check into your state's requirements.  Some states may allow for an equivalent degree, like architecture. 

good grief,...an inferior decorator?

There were a few states where licensure for IDs was struck down for being unconstitutional...I know there are several states that protect the title "licensed ID" but "ID" itself cannot be a protected title as courts have ruled.  There may be some instances in certain states where a licensed ID is required ...like for public works...just check the laws of the land that you want to work in before spending time and money on unnecessary pursuits. 

Actually interior designers also design the interiors of boats, trains, airplanes, cars, as well as buildings. Any well designed building can have a number of interiors through the years as its function changes over its lifetime. Interior design is also tied up with ergonomics in much, if not all, all they design so it is not a fluff discipline by any means. It is certainly not something you would leave to an architect who as little or no interest in doing it well.

Can interior designer can do the work of architecture ?????

Like just designing the exterior part of buildings ...

Can you become an interior designer from an architecture degree? Yes but it would be a specific domain within the breadth of the field of architecture that you would have to specialize in but possible.

Can you do architecture if you are an interior designer? Well, that can be a more convoluted answer. Can you design buildings? Yes.... exempt buildings. Should you? Not unless you have the knowledge and skills to do so competently. To do "architecture" (as in the work that requires a license in the U.S.) then you need to get licensed as an architect (in the U.S.... giving a geographical/jurisdictional context because things get more complicated in the world) so in every pathway to licensure in the U.S., you have to have recognized education/experience that qualifies as well as take a series of exams. Interior design education core does not cover a number of required subject areas to qualify fully and there are various factors where you have to make up the deficiency in the licensure path.(at least some of it)

There is generalities given but the long and thorough answer is not something I'm going to go into.

Depends on your jurisdiction. Some have governing bodies for Interior Designers. Ask them.

Here in Canada Architects can be a Interior designer without a  bachelor degree in Interior design. A  Interior Designer can’t be a Architect without the masters in Architect.

Can you become an interior designer with a degree in architecture? In all but maybe a couple states, yes. Most states don't even regulate the title 'interior designer'. However, there are states that have a license/registration/certification for being conferred a title such as "certified interior designer" or a few other examples. In that case, it can be more complicated. 

In Oregon, interior design and interior designer is all unregulated from licensure requirements so the answer is.... profoundly.... a yes *if* you are any way or form competent at designing interior in any resemblance of a professional standard. If not, then you have a more profound problem then title issues or practicing interior design.

it’s very clear that there are few people here who do not know crap that they are talking about. If you want to inform others get your facts straight before you say anything. All it will do is make your self look stupid and uninformed.