Just know the Difference between Row Number, Rank and Dense rank

              Difference between Row Number, Rank and Dense rank

Which one to use? 

This is very common question in the minds of SQL newbie's. Lets take 1 simple example to understand the difference between 3. 

First lets create some sample data :

- create table CREATE TABLE Salaries ( Names VARCHAR(1), SalarY INT ) -- insert data INSERT INTO Salaries SELECT 'A',5000 UNION ALL SELECT 'B',5000 UNION ALL SELECT 'C',3000 UNION ALL SELECT 'D',4000 UNION ALL SELECT 'E',6000 UNION ALL SELECT 'F',10000 -- Test the data Select Names, Salary From Salaries

Now lets query the table to get the salaries of all employees with their salary in descending order.
For that I'll write a query like this :

Select  names , salary                ,row_number () OVER (ORDER BY salary DESC) as ROW_NUMBER               ,rank () OVER (ORDER BY salary DESC) as RANK              ,dense_rank () OVER (ORDER BY salary DESC) as DENSE_RANK From  salaries

---------------Output

NAMES	SALARY	ROW_NUMBER	RANK	DENSE_RANK
F	10000	1	1	1
E	6000	2	2	2
A	5000	3	3	3
B	5000	4	3	3
D	4000	5	5	4
C	3000	6	6	5

Interesting Names in the result are employee A, B and D. Row_number assign different number to them.
Rank and Dense_rank both assign same rank to A and B.
But interesting thing is what RANK and DENSE_RANK assign to next row?
Rank assign 5 to the next row, while dense_rank assign 4.
The numbers returned by the DENSE_RANK function do not have gaps and always have consecutive  ranks.  The RANK function does not always return consecutive integers.  The ORDER BY clause  determines the sequence in which the rows are assigned their unique ROW_NUMBER within a specified  partition.
So question is which one to use?
Its all depends on your requirement and business rule you are following.
1. Row_number to be used only when you just want to have serial number on result set. It is not as intelligent     as RANK and DENSE_RANK.
2. Choice between RANK and DENSE_RANK depends on business rule you are following. Rank leaves       the gaps between number when it sees common values in 2 or more rows. DENSE_RANK don't leave        any gaps between ranks.
So while assigning the next rank to the row RANK will consider the total count of rows before that row and    DESNE_RANK will just give next rank according to the value.
So If you are selecting employee’s rank according to their salaries you should be using DENSE_RANK and   if you are ranking students according to there marks you should be using RANK
  (Though it is not mandatory, depends on your requirement.)