Need to choose which career either Network Administrator or Database Administrator ?

They have different opportunities. There are a lot more 'network administrator' positions available than there are 'database administrators'. So there are advantages and disadvantages to both. Database administrator positions are harder to find, but if you're good at the job, you'll be sought after. Network administrator positions are more plentiful, so if you're just 'decent', you'll be more likely to find a position. The more important questions are a) what will you be better at and b) what do you enjoy more?