This topic describes the syntax of window functions and provides examples on how to use window functions.
Summary
Aggregate functions calculate the single result for a group of rows, and window functions calculate the result for each row in a group. A window function has three elements: partition, order, and frame. For more information, see Window Function Concepts and Syntax.
function over (
[partition by partition_expression]
[order by order_expression]
[frame]
)
 Partition: The partition element is defined by the PARTITION BY clause. The PARTITION BY clause separates rows into partitions. If you do not specify the PARTITION BY clause, all rows are treated as a single partition.
 Order: The order element is defined by the ORDER BY clause. The ORDER BY clause sorts
rows in all partitions.
Note If you use the ORDER BY clause to sort rows on fields that have the same value, the order of these rows is nondeterministic. You can include additional fields in the ORDER BY clause to obtain the expected order of these rows. Example:
order by request_time, request_method
.  Frame: The frame element is defined by the FRAME clause. The FRAME clause specifies
a subset of each partition. A frame further refines the rows in each partition. You
cannot specify the FRAME clause for ranking functions. Syntax of the FRAME clause:
{ rows  range} { frame_start  frame_between }
. Example:range between unbounded preceding and unbounded following
. For more information, see Window Function Frame Specification.
Window functions
Category  Function  Syntax  Description 

Aggregate functions  Aggregate functions  None  You can use all aggregate functions as window functions. For more information about aggregate functions, see Aggregate function. 
Ranking functions  cume_dist function  cume_dist()  Calculate the cumulative distribution of each value in a partition. The result is obtained by using division. The numerator is the number of rows whose field values are smaller or equal to the field value of the specified row. The specified row is also counted. The denominator is the total number of rows in the partition. The calculation is based on the order of the rows in the partition. Value range: (0,1]. 
dense_rank function  dense_rank()  Calculates the rank of each value in a partition. Rows that have the same field value are assigned the same rank. The ranks are consecutive. For example, if two rows have the same rank of 1, the rank of the next row is 2.  
ntile function  ntile(n)  Divide the rows in each partition into the number of groups specified by the N parameter.  
percent_rank function  percent_rank()  Calculates the percentage ranking of each row in a partition.  
rank function  rank()  Calculate the rank of each row in a partition. Rows that have the same field value are assigned the same rank. The ranks are not consecutive. For example, if two rows have the same rank of 1, the rank of the next row is 3.  
row_number function  row_number()  Calculate the rank of each row in a partition. The ranks are unique, and start from 1. Rows that have the same value are assigned consecutive ranks. For example, three rows with the same field value are assigned the ranks 1, 2, and 3.  
Offset functions  first_value function  first_value(x)  Returns the value of the specified field in the first row of each partition. 
last_value function  last_value(x)  Returns the value of the specified field in the last row of each partition.  
lag function  lag(x,offset,defaut_value)  Returns the value of the specified field in the row that is at the specified offset before the current row of each partition. The offset is specified by the offset parameter. If no row exists at the specified offset before the current row, the value that is specified by the defaut_value parameter is returned for the partition.  
lead function  lead(x,offset,defaut_value)  Returns the value of the specified field in the row that is at the specified offset after the current row of each partition. The offset is specified by the offset parameter. If no row exists at the specified offset after the current row, the value that is specified by the defaut_value is returned for the partition.  
nth_value function  nth_value(x, offset)  Returns the value of the specified field in the row that is at the specified offset from the beginning of each partition. The offset is specified by the offset parameter. 
Aggregate functions
You can use all aggregate functions as window functions. For more information about aggregate functions, see Aggregate function. The following example shows how to use the sum function as a window function.
Syntax
sum() over (
[partition by partition_expression]
[order by order_expression]
[frame]
)
Parameters
Parameter  Description 

partition by partition_expression  Specifies how the rows are partitioned based on the value of the partition_expression parameter. 
order by order_expression  Specifies how the rows in each partition are ordered based on the value of the order_expression parameter. 
frame  Specifies a subset of each partition. Example: range between unbounded preceding and unbounded following .

Return value type
The double data type.
Examples
Calculate the percentage of each employee salary in each department.
 Query statement
*  SELECT department, staff_name, salary, round ( salary * 1.0 / sum(salary) over(partition by department), 3) AS salary_percentage
 Query result
cume_dist function
Calculates the cumulative distribution of each value in a partition. The result is obtained by using division. The numerator is the number of rows whose field values are smaller or equal to the field value of the specified row. The specified row is also counted. The denominator is the total number of rows in the partition. The calculation is based on the order of the rows in the partition. Value range: (0,1].
Syntax
cume_dist() over (
[partition by partition_expression]
[order by order_expression]
)
Parameters
Parameter  Description 

partition by partition_expression  Specifies how the rows are partitioned based on the value of the partition_expression parameter. 
order by order_expression  Specifies how the rows in each partition are ordered. The rows are ordered based on the value of the order_expression parameter. 
Return value type
The double data type.
Examples
Calculate the cumulative distribution of the size of each object in an OSS bucket named bucket00788.
 Query statement
bucket=bucket00788  select object, object_size, cume_dist() over ( partition by object order by object_size ) as cume_dist from osslogstore
 Query result
dense_rank function
Calculates the rank of each value in a partition. Rows that have the same field value are assigned the same rank. The ranks are consecutive. For example, if two rows have the same rank of 1, the rank of the next row is 2.
Syntax
dense_rank() over (
[partition by partition_expression]
[order by order_expression]
)
Parameters
Parameter  Description 

partition by partition_expression  Specifies how the rows are partitioned based on the value of the partition_expression parameter. 
order by order_expression  Specifies how the rows in each partition are ordered based on the value of the order_expression parameter. 
Return value type
The bigint data type.
Examples
Calculate the rank of each employee salary in each department.
 Query statement
*  select department, staff_name, salary, dense_rank() over( partition by department order by salary desc ) as salary_rank order by department, salary_rank
 Query result
ntile function
Divides the rows in each partition into a number of groups. The number of groups is specified by the N parameter.
Syntax
ntile(n) over (
[partition by partition_expression]
[order by order_expression]
)
Parameters
Parameter  Description 

n  Specifies the number of groups. 
partition by partition_expression  Specifies how the rows are partitioned based on the value of the partition_expression parameter. 
order by order_expression  Specifies how the rows in each partition are ordered based on the value of the order_expression parameter. 
Return value type
The bigint data type.
Examples
Divide the rows in each partition into three groups.
 Query statement
object=245da918c.model  select object, object_size, ntile(3) over ( partition by object order by object_size ) as ntile from osslogstore
 Query result
percent_rank function
Calculates the percentage ranking of each row in a partition. The calculation formula
is (rank  1)/(total_rows  1)
. In the formula, rank represents the rank of the current row, and total_rows represents
the total number of rows in a partition.
Syntax
percent_rank() over (
[partition by partition_expression]
[order by order_expression]
)
Parameters
Parameter  Description 

partition by partition_expression  Specifies how the rows are partitioned based on the value of the partition_expression parameter. 
order by order_expression  Specifies how the rows in each partition are ordered based on the value of the order_expression parameter. 
Return value type
The double data type.
Examples
Calculate the percentage ranking of the size of each OSS object.
 Query statement
object=245da918c3e2dd9dc9cb4d9283b%2F555e2441b6a4c7f094099a6dba8e7a5f.model select object, object_size, percent_rank() over ( partition by object order by object_size ) as ntile FROM osslogstore
 Query result
rank function
Calculates the rank of each row in a partition. Rows that have the same field value are assigned the same rank. The ranks are not consecutive. For example, if two rows have the same rank of 1, the rank of the next row is 3.
Syntax
rank() over (
[partition by partition_expression]
[order by order_expression]
)
Parameters
Parameter  Description 

partition by partition_expression  Specifies how the rows are partitioned based on the value of the partition_expression parameter. 
order by order_expression  Specifies how the rows in each partition are ordered based on the value of the order_expression parameter. 
Return value type
The bigint data type.
Examples
Calculate the rank of each employee salary in each department.
 Query statement
*  select department, staff_name, salary, rank() over( partition by department order by salary desc ) as salary_rank order by department, salary_rank
 Query result
row_number function
Calculate the rank of each row in a partition. The ranks are unique, and start from 1.
Syntax
row_number() over (
[partition by partition_expression]
[order by order_expression]
)
Parameters
Parameter  Description 

partition by partition_expression  Specifies how the rows are partitioned based on the value of the partition_expression parameter. 
order by order_expression  Specifies how the rows in each partition are ordered. The rows are ordered based on the value of the order_expression parameter. 
Return value type
The bigint data type.
Examples
Calculate the rank of each employee on the salary in each department.
 Query statement
*  select department, staff_name, salary, row_number() over( partition by department order by salary desc ) as salary_rank order by department, salary_rank
 Query result
first_value function
Returns the value of the specified field in the first row of each partition.
Syntax
first_value(x) over (
[partition by partition_expression]
[order by order_expression]
[frame]
)
Parameters
Parameter  Description 

x  The field name. The field can be of any data type. 
partition by partition_expression  Specifies how the rows are partitioned based on the value of the partition_expression parameter. 
order by order_expression  Specifies how the rows in each partition are ordered based on the value of the order_expression parameter. 
frame  Specifies a subset of each partition. Example: range between unbounded preceding and unbounded following .

Return value type
The data type is the same as the data type of the x parameter.
Examples
Return the minimum size of each object in the specified OSS bucket.
 Query statement
bucket :bucket90  select object, object_size, first_value(object_size) over ( partition by object order by object_size range between unbounded preceding and unbounded following ) as last_value from osslogstore
 Query result
last_value function
Returns the value of the specified field in the last row of each partition.
Syntax
last_value(x) over (
[partition by partition_expression]
[order by order_expression]
[frame]
)
Parameters
Parameter  Description 

x  The field name. The field can be of any data type. 
partition by partition_expression  Specifies how the rows are partitioned based on the value of the partition_expression parameter. 
order by order_expression  Specifies how the rows in each partition are ordered based on the value of the order_expression parameter. 
frame  Specifies a subset of each partition. Example: range between unbounded preceding and unbounded following .

Return value type
The data type is the same as the data type of the x parameter.
Examples
Return the maximum size of each object in the specified OSS bucket.
 Query statement
bucket :bucket90  select object, object_size, last_value(object_size) over ( partition by object order by object_size range between unbounded preceding and unbounded following ) as last_value from osslogstore
 Query result
lag function
Returns the value of the specified field in the row that is at the specified offset before the current row in each partition. The offset is specified by the offset parameter.
Syntax
lag(x,offset,defaut_value) over (
[partition by partition_expression]
[order by order_expression]
[frame]
)
Parameters
Parameter  Description 

x  The field name. The field can be of any data type. 
offset  The offset before the current row in a partition. If the value of the offset parameter is 0, the value of the specified field in the current row is returned. 
defaut_value  If no row exists at the specified offset before the current row, the value of the defaut_value parameter is returned. 
partition by partition_expression  Specifies how the rows are partitioned based on the value of the partition_expression parameter. 
order by order_expression  Specifies how the rows in each partition are ordered based on the value of the order_expression parameter. 
frame  Specifies a subset of each partition. Example: range between unbounded preceding and unbounded following .

Return value type
The data type is the same as the data type of the x parameter.
Examples
Count the daily unique visitors (UVs) to your website and calculates the percentage of the increase in UVs over the previous day.
 Query statement
*  select day, UV, UV * 1.0 /(lag(UV, 1, 0) over()) as diff_percentage from ( select approx_distinct(client_ip) as UV, date_trunc('day', __time__) as day from log group by day order by day asc )
 Query result
lead function
Returns the value of the specified field in the row that is at the specified offset after the current row in each partition. The offset is specified by the offset parameter.
Syntax
lead(x,offset,defaut_value) over (
[partition by partition_expression]
[order by order_expression]
[frame]
)
Parameters
Parameter  Description 

x  The field name. The field can be of any data type. 
offset  The offset after the current row in a partition. If the value of the offset parameter is 0, the value of the specified field in the current row is returned. 
defaut_value  If no row exists at the specified offset after the current row, the value of the defaut_value parameter is returned. 
partition by partition_expression  Specifies how the rows are partitioned based on the value of the partition_expression parameter. 
order by order_expression  Specifies how the rows in each partition are ordered based on the value of the order_expression parameter. 
frame  Specifies a subset of each partition. Example: range between unbounded preceding and unbounded following .

Return value type
The data type is the same as the data type of the x parameter.
Examples
Count the hourly unique visitors (UVs) to your website on 20210826 and calculates the difference in percentage between UVs of two consecutive hours.
 Query statement
*  select time, UV, UV * 1.0 /(lead(UV, 1, 0) over()) as diff_percentage from ( select approx_distinct(client_ip) as uv, date_trunc('hour', __time__) as time from log group by time order by time asc )
 Query result
nth_value function
Returns the value of the specified field in the row that is at the specified offset from the beginning of each partition. The offset is specified by the offset parameter.
Syntax
nth_value(x,offset) over (
[partition by partition_expression]
[order by order_expression]
[frame]
)
Parameters
Parameter  Description 

x  The field name. The field can be of any data type. 
offset  The offset from the beginning of a partition. 
partition by partition_expression  Specifies how the rows are partitioned based on the value of the partition_expression parameter. 
order by order_expression  Specifies how the rows in each partition are ordered based on the value of the order_expression parameter. 
frame  Specifies a subset of each partition. Example: range between unbounded preceding and unbounded following .

Return value type
The data type is the same as the data type of the x parameter.
Examples
Return the employees whose salary is the second highest in each department.
 Query statement
*  select department, staff_name, salary, nth_value(staff_name, 2) over( partition by department order by sallary desc range between unbounded preceding and unbounded following ) as second_highest_sallary from log
 Query result