Since you have done B.Sc. it is better to do a master degree course that is application of computers. Why do you want to do a diploma? You are eligible for a 2 year degree course. In any case, a diploma course is 2 years long, which is same as that of MCA or Master's in Computer Applications. So it is better to do MCA. The best university for pursuing MCA and a job in parallel is IGNOU university. This is a distance university, so you can attend the classes after your office hours. You can manage both your job and education. Also the classes in most of the IGNOU regional centers are held on Saturdays and Sundays. Government offices are closed on those days.
MCA is a technical course that will help you get job positions of software developer, quality analyzer, software tester, computer trainer and even computer professor in colleges. So there are lots of jobs to be availed after MCA, provided you have the necessary score to be selected for the job interview. Lastly, you can still complete a B.Tech or B.E. after your graduation. But what is the use of it? It is only a wastage of another 4 years, where you could have done a master's degree for 2 years. In fact, AICTE holds MCA course equivalent to be an undergraduate course in engineering. Think about it.
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