grammar ：ROW_NUMBER() OVER(PARTITION BY COLUMN ORDER BY COLUMN)

To put it simply row_number() from 1 Start , Return a number for each grouped record , there ROW_NUMBER() OVER (ORDER BY xlh DESC) First of all. xlh Column descending , No more items after descending xlh The record returns a sequence number .
Example ：
xlh           row_num
1700              1
1500              2
1085              3
710                4

row_number() OVER (PARTITION BY COL1 ORDER BY COL2) Express basis COL1 grouping , Within the group according to COL2 Sort , The value calculated by this function represents the sequence number of each group after internal sorting （ The only continuous )

example ：

Initialization data

create table employee (empid int ,deptid int ,salary decimal(10,2))
insert into employee values(1,10,5500.00)
insert into employee values(2,10,4500.00)
insert into employee values(3,20,1900.00)
insert into employee values(4,20,4800.00)
insert into employee values(5,40,6500.00)
insert into employee values(6,40,14500.00)
insert into employee values(7,40,44500.00)
insert into employee values(8,50,6500.00)
insert into employee values(9,50,7500.00)

The data shows

empid       deptid      salary
----------- ----------- ---------------------------------------
1           10          5500.00
2           10          4500.00
3           20          1900.00
4           20          4800.00
5           40          6500.00
6           40          14500.00
7           40          44500.00
8           50          6500.00
9           50          7500.00

demand ： Group by Department , Display the salary level of each department

Expected results ：

empid       deptid      salary                                  rank
----------- ----------- --------------------------------------- --------------------
1           10          5500.00                                 1
2           10          4500.00                                 2
4           20          4800.00                                 1
3           20          1900.00                                 2
7           40          44500.00                                1
6           40          14500.00                                2
5           40          6500.00                                 3
9           50          7500.00                                 1
8           50          6500.00                                 2

SQL Script ：

SELECT *, Row_Number() OVER (partition by deptid ORDER BY salary desc) rank FROM employee

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