How to count number of entries depending on the string in a field in sql

Try This:
 

DateTime dtmMechCode = Convert.ToDateTime(DateTime.Now);

 

string strMechCode = Convert.ToString(dtmMechCode).Replace("/", "").Replace(":", "").Replace(" ", "").Replace("AM", "").Replace("PM", "");

 

string UniqueId = "RE" + strMechCode;