1. ROW_NUMBER()
2. RANK()
3. DENSE_RANK()

All of these three functions are used to calculate the Id of row but in different way.

I am using the below script for examples

CREATE TABLE Marks
(
SubjectId INT,
Marks INT
)
INSERT INTO Marks VALUES(1,100),
                         (2,90),
						 (3,80),
						 (4,70),
						 (5,70),
						 (6,50),
						 (7,40),
						 (8,40),
						 (9,30)

1. ROW_NUMBER():-

    This function will assign the unique Id to each row of result.

Please see the result in above image. ROW_NUMBER() has assigned unique Id to each record

2) Rank() :-

 This function works in same way as ROW_NUMBER() except the "equal" records are ranked the same. This will  leave the gap of between the same values. please see the below result

As we can see Marks with value 70 is assigned the same rank 4 but for the next value it gave rank 6 it skipped the rank 5. Same case is for value 40

3) DENSE_RANK() :-

This function is same like RANK() but it does not leave the gap when assigning the rank. it is “dense”. Please refer the below image