When we write SQL code, as long as there is sorting, the first thing that comes to mind must be ORDER BY, so many friends think that sorting is so simple.
Today I will introduce four sorting functions that you don’t use very often. They are the four good brothers ROW_NUMBER(), RANK(), DENSE_RANK(), and NTILE() that are often used in sorting.
Let's first create a test data table Scores
WITH t AS
(SELECT 1 StuID,70 Score
UNION ALL
SELECT 2,85
UNION ALL
SELECT 3,85
UNION ALL
SELECT 4,80
UNION ALL
SELECT 5,74
)
SELECT * INTO Scores FROM t;
SELECT * FROM Scores
The result is as follows:
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1、ROW_NUMBER()
Definition : The function of the ROW_NUMBER() function is to sort the data queried by SELECT, and add a serial number to each piece of data. It cannot be used to rank students' grades. It is generally used for page-by-page queries, such as querying the top 10 queries 10-100 student.
1.1 Sorting students' grades
example
SELECT
ROW_NUMBER() OVER (ORDER BY SCORE DESC) AS [RANK],*
FROM Scores;
(Hint: you can slide the code left and right)
The result is as follows:
Here RANK is the order after the ranking of each student, and DESC is reversed according to Score
1.2 Obtain the result information of the second place
SELECT * FROM (
SELECT ROW_NUMBER() OVER (ORDER BY SCORE DESC) AS [RANK],*
FROM Scores
) t WHERE t.RANK=2;
result:
The idea used here is to add a layer of SELECT
WHERE t.RANK>=1 AND t.RANK<=3 to the original sql with the idea of pagination query. Is it just to get the grade information of the top three students?
2、RANK()
Definition: The RANK() function, as the name suggests, can rank a certain field. What is the difference between this and ROW_NUMBER()? ROW_NUMBER() is sorting. When there are students with the same grade, ROW_NUMBER() will sort them in turn. Their serial numbers are different, but Rank() is different. If the same appears, their rankings are the same. Let's see an example:
example
SELECT ROW_NUMBER() OVER (ORDER BY SCORE DESC) AS [RANK],*
FROM Scores;
SELECT RANK() OVER (ORDER BY SCORE DESC) AS [RANK],*
FROM Scores;
result:
Above is the result of the ROW_NUMBER() function, and below is the result of the RANK() function.
When there are two students with the same grade, there will be a change. RANK() is 1-1-3-4-5, while ROW_NUMBER() is still 1-2-3-4-5, this is the difference between RANK() and ROW_NUMBER()
3、DENSE_RANK()
Definition : The DENSE_RANK() function is also a ranking function. It is similar to RANK() and ranks fields. So what is the difference between it and RANK()? Especially in the case of the same score, DENSE_RANK() ranking is continuous, RANK() is a jumping ranking, and the ranking function used in general is RANK() Let's look at an example:
example
SELECT
RANK() OVER (ORDER BY SCORE DESC) AS [RANK],*
FROM Scores;
SELECT
DENSE_RANK() OVER (ORDER BY SCORE DESC) AS [RANK],*
FROM Scores;
result:
Above is the result of RANK(), below is the result of DENSE_RANK()
4、NTILE()
Definition: The NTILE() function is to distribute the rows in the ordered partition to the specified number of groups, each group has a number, and the number starts from 1, just like what we call 'partition', it is divided into several districts, one district How many will there be.
SELECT NTILE(1) OVER (ORDER BY SCORE DESC) AS \[RANK\],\* FROM Scores;
SELECT NTILE(2) OVER (ORDER BY SCORE DESC) AS \[RANK\],\* FROM Scores;
SELECT NTILE(3) OVER (ORDER BY SCORE DESC) AS \[RANK\],\* FROM Scores;
result:
It is to divide the queried records into equal partitions according to the parameters in the NTILE function.
The introduction of these brothers is over, if you think it is good, remember to help forward and share it with more people