I was trying to find the answer to the same question and below explanation gives you better understanding i hope.   The merge join works by simultaneously reading and comparing the two sorted inputs one row at a time.  At each step, we compare the next row from each input.  If the rows are equal, we output a joined row and continue.  If the rows are not equal, we discard the lesser of the two inputs and continue.  Since the inputs are sorted, we know that we are discarding a row that is less than any of the remaining rows in either input and, thus, can never join.   We can express the algorithm in pseudo-code as:   get first row R1 from input 1 get first row R2 from input 2 while not at the end of either input     begin         if R1 joins with R2             begin                 return (R1, R2)                 get next row R2 from input 2             end         else if R1 < R2             get next row R1 from input 1         else             get next row R2 from input 2     end   Source :https://blogs.msdn.microsoft.com/craigfr/2006/08/03/merge-join/ ... View more