HI Nitesh !
The two posts which you mentioned Deputy Collector (DC) and Sub-Divisional Magistrate (SDM) are not different. SDM is a post given to administrative officer below level of district. A district is divided into several Sub-Divisions (tehsils) and the head of those sub-divisions is called as SDM. The Deputy Collector can get several ranks in district level. SDM is one such post a Deputy Collector can acquire.
Hope this helps !
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