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Window function

Background

The window function is a special class of built-in functions. Similar to the aggregation function, it also does calculations on multiple input rows to get a single data value. The difference is that the window function processes the input data within a specific window, rather than using the "group by" method. The data in each window can be sorted and grouped using the over() clause. The window function computes a separate value for each row, rather than computing one value for each group. This flexibility allows users to add additional columns to the select clause and further filter the result set. The window function can only appear in the select list and the outermost position of a clause. It takes effect at the end of the query, that is, after the join, where, and group by operations are performed. The window function is often used to analyze trends, calculate outliers, and perform bucketing analyses on large-scale data.

Usage

Syntax of the window function:

function(args) OVER(partition_by_clause order_by_clause [window_clause])
partition_by_clause ::= PARTITION BY expr [, expr ...]
order_by_clause ::= ORDER BY expr [ASC | DESC] [, expr [ASC | DESC] ...]

Function

Currently supported Functions include:

  • MIN(), MAX(), COUNT(), SUM(), AVG()
  • FIRST_VALUE(), LAST_VALUE(), LEAD(), LAG()
  • ROW_NUMBER(), RANK(), DENSE_RANK()

PARTITION BY clause

The Partition By clause is similar to Group By. It groups the input rows by one or more specified columns. Rows with the same value are grouped together.

ORDER BY clause

The Order By clause is basically the same as the outer Order By. It defines the order of the input rows. IfPartition By is specified, Order By defines the order within each Partition grouping. The only difference is that Order By n (n is a positive integer) in the OVER clause is equivalent to no operation, whereas the n in the outer Order By indicates sorting by the nth column.

Example 1:

This example shows adding an id column to the select list with values of 1, 2, 3, etc., sorted by the date_and_time column in the events table.

SELECT row_number() OVER (ORDER BY date_and_time) AS id,
    c1, c2, c3, c4
FROM events;

Window Clause

The window clause is used to specify a range of rows for operations ( the previous and later lines based on the current line). It supports the following syntaxes – AVG(), COUNT(), FIRST_VALUE(), LAST_VALUE() and SUM(). For MAX() and MIN(), the window clause can specify the start to UNBOUNDED PRECEDING.

Syntax:

ROWS BETWEEN [ { m | UNBOUNDED } PRECEDING | CURRENT ROW] [ AND [CURRENT ROW | { UNBOUNDED | n } FOLLOWING] ]

Example 2:

Suppose we have the following stock data, the stock symbol is JDR, and the closing price is the daily closing price.

create table stock_ticker (
    stock_symbol string,
    closing_price decimal(8,2),
    closing_date timestamp);

-- ...load some data...

select *
from stock_ticker
order by stock_symbol, closing_date

The raw data was shown as follows:

+--------------+---------------+---------------------+
| stock_symbol | closing_price | closing_date        |
+--------------+---------------+---------------------+
| JDR          | 12.86         | 2014-10-02 00:00:00 |
| JDR          | 12.89         | 2014-10-03 00:00:00 |
| JDR          | 12.94         | 2014-10-04 00:00:00 |
| JDR          | 12.55         | 2014-10-05 00:00:00 |
| JDR          | 14.03         | 2014-10-06 00:00:00 |
| JDR          | 14.75         | 2014-10-07 00:00:00 |
| JDR          | 13.98         | 2014-10-08 00:00:00 |
+--------------+---------------+---------------------+

This query uses the window function to generate the moving_average column whose value is the 3-day (previous day, current day, and next day) average stock price. The first day does not have the value of its previous day, and the last day does not have the value of the day after, so these two rows only calculate the average value of two days. Here Partition By does not take effect, because all the data is JDR data. However, if there is other stock information, Partition By will ensure that the window function is operated within each Partition.

select stock_symbol, closing_date, closing_price,
    avg(closing_price)
        over (partition by stock_symbol
              order by closing_date
              rows between 1 preceding and 1 following
        ) as moving_average
from stock_ticker;

The following data is obtained:

+--------------+---------------------+---------------+----------------+
| stock_symbol | closing_date        | closing_price | moving_average |
+--------------+---------------------+---------------+----------------+
| JDR          | 2014-10-02 00:00:00 | 12.86         | 12.87          |
| JDR          | 2014-10-03 00:00:00 | 12.89         | 12.89          |
| JDR          | 2014-10-04 00:00:00 | 12.94         | 12.79          |
| JDR          | 2014-10-05 00:00:00 | 12.55         | 13.17          |
| JDR          | 2014-10-06 00:00:00 | 14.03         | 13.77          |
| JDR          | 2014-10-07 00:00:00 | 14.75         | 14.25          |
| JDR          | 2014-10-08 00:00:00 | 13.98         | 14.36          |
+--------------+---------------------+---------------+----------------+

Function Examples

This section describes the window functions supported in StarRocks.

AVG()

Syntax:

AVG(expression) [OVER (*analytic_clause*)]

Example 3:

Calculate the x-average of the current row and each row before and after it.

select x, property,
    avg(x)
        over (
            partition by property
            order by x
            rows between 1 preceding and 1 following
        ) as 'moving average'
from int_t
where property in ('odd','even');
+----+----------+----------------+
| x  | property | moving average |
+----+----------+----------------+
| 2  | even     | 3              |
| 4  | even     | 4              |
| 6  | even     | 6              |
| 8  | even     | 8              |
| 10 | even     | 9              |
| 1  | odd      | 2              |
| 3  | odd      | 3              |
| 5  | odd      | 5              |
| 7  | odd      | 7              |
| 9  | odd      | 8              |
+----+----------+----------------+

COUNT()

Syntax:

COUNT(expression) [OVER (analytic_clause)]

Example 4:

Count the occurrence of x from the current row to the first row.

select x, property,
    count(x)
        over (
            partition by property
            order by x
            rows between unbounded preceding and current row
        ) as 'cumulative total'
from int_t where property in ('odd','even');
+----+----------+------------------+
| x  | property | cumulative count |
+----+----------+------------------+
| 2  | even     | 1                |
| 4  | even     | 2                |
| 6  | even     | 3                |
| 8  | even     | 4                |
| 10 | even     | 5                |
| 1  | odd      | 1                |
| 3  | odd      | 2                |
| 5  | odd      | 3                |
| 7  | odd      | 4                |
| 9  | odd      | 5                |
+----+----------+------------------+

DENSE_RANK()

The DENSE_RANK() function is used to represent rankings. Unlike RANK(), DENSE_RANK()does not have vacant numbers. For example, if there are two tied 1s, the third number of DENSE_RANK() is still 2, whereas the third number of RANK() is 3.

Syntax:

DENSE_RANK() OVER(partition_by_clause order_by_clause)

The following example shows the ranking of column x according to the property column grouping.

select x, y,
    dense_rank()
        over (
            partition by x
            order by y
        ) as rank
from int_t;
+---+---+------+
| x | y | rank |
+---+---+------+
| 1 | 1 | 1    |
| 1 | 2 | 2    |
| 1 | 2 | 2    |
| 2 | 1 | 1    |
| 2 | 2 | 2    |
| 2 | 3 | 3    |
| 3 | 1 | 1    |
| 3 | 1 | 1    |
| 3 | 2 | 2    |
+---+---+------+

NTILE

NTILE() function divides the sorted rows in a partition by the specified number of num_buckets as equally as possible, stores the divided rows in the respective buckets, starting from 1 [1, 2, ..., num_buckets], and returns the bucket number that each row is in.

About the size of the bucket:

  • If the row counts can be divided by the specified number of num_buckets exactly, all the buckets will be of the same size.
  • If the row counts cannot be divided by the specified number of num_buckets exactly, there will be buckets of two different sizes. The difference between sizes is 1. The buckets with more rows will be listed ahead of the one with fewer rows.

Syntax:

NTILE (num_buckets) OVER (partition_by_clause order_by_clause)

num_buckets: Number of the buckets to be created. The value must be a constant positive integer whose maximum is 2^63 - 1.

Window clause is not allowed in NTILE() function

NTILE() function returns BIGINT type of data.

Example:

The following example divides all rows in the partition into 2 buckets.

select id, x, y,
    ntile(2)
        over (
            partition by x
            order by y
        ) as bucket_id
from t1;
+------+------+------+-----------+
| id   | x    | y    | bucket_id |
+------+------+------+-----------+
|    1 |    1 |   11 |         1 |
|    2 |    1 |   11 |         1 |
|    3 |    1 |   22 |         1 |
|    4 |    1 |   33 |         2 |
|    5 |    1 |   44 |         2 |
|    6 |    1 |   55 |         2 |
|    7 |    2 |   66 |         1 |
|    8 |    2 |   77 |         1 |
|    9 |    2 |   88 |         2 |
|   10 |    3 |   99 |         1 |
+------+------+------+-----------+

As the above example shown, when num_buckets is 2:

  • Rows of No.1 to No.6 were classified into the first partition; rows of No.1 to No.3 were stored in the first bucket, and rows of No.4 to No.6 were stored in the second one.
  • Rows of No.7 to No.9 were classified into the second partition; rows of No.7 and No.8 were stored in the first bucket, and row No.9 was stored in the second one.
  • Row No.10 was classified into the third partition and stored in the first bucket.

FIRST_VALUE()

FIRST_VALUE() returns the first value of the window range.

Syntax:

FIRST_VALUE(expr) OVER(partition_by_clause order_by_clause [window_clause])

For example 5:

We have the following data:

 select name, country, greeting
 from mail_merge;
+---------+---------+--------------+
| name    | country | greeting     |
+---------+---------+--------------+
| Pete    | USA     | Hello        |
| John    | USA     | Hi           |
| Boris   | Germany | Guten tag    |
| Michael | Germany | Guten morgen |
| Bjorn   | Sweden  | Hej          |
| Mats    | Sweden  | Tja          |
+---------+---------+--------------+

Use FIRST_VALUE() to return the first greeting value in each grouping, based on the country grouping.

select country, name,
    first_value(greeting)
        over (
            partition by country
            order by name, greeting
        ) as greeting
from mail_merge;
+---------+---------+-----------+
| country | name    | greeting  |
+---------+---------+-----------+
| Germany | Boris   | Guten tag |
| Germany | Michael | Guten tag |
| Sweden  | Bjorn   | Hej       |
| Sweden  | Mats    | Hej       |
| USA     | John    | Hi        |
| USA     | Pete    | Hi        |
+---------+---------+-----------+

LAG()

The LAG() method is used to calculate the value of rows ahead of the current row.

Syntax:

LAG (expr, offset, default) OVER (partition_by_clause order_by_clause)

Example 6:

Calculate the closing price of the previous day

select stock_symbol, closing_date, closing_price,
    lag(closing_price,1, 0) over
    (
        partition by stock_symbol
        order by closing_date
    ) as "yesterday closing"
from stock_ticker
order by closing_date;
+--------------+---------------------+---------------+-------------------+
| stock_symbol | closing_date        | closing_price | yesterday closing |
+--------------+---------------------+---------------+-------------------+
| JDR          | 2014-09-13 00:00:00 | 12.86         | 0                 |
| JDR          | 2014-09-14 00:00:00 | 12.89         | 12.86             |
| JDR          | 2014-09-15 00:00:00 | 12.94         | 12.89             |
| JDR          | 2014-09-16 00:00:00 | 12.55         | 12.94             |
| JDR          | 2014-09-17 00:00:00 | 14.03         | 12.55             |
| JDR          | 2014-09-18 00:00:00 | 14.75         | 14.03             |
| JDR          | 2014-09-19 00:00:00 | 13.98         | 14.75             |
+--------------+---------------------+---------------+-------------------+

LAST_VALUE()

LAST_VALUE() returns the last value of the window range. It is the opposite of FIRST_VALUE().

Syntax:

LAST_VALUE(expr) OVER(partition_by_clause order_by_clause [window_clause])

Use the data form the example 6:

select country, name,
    last_value(greeting)
        over (
            partition by country
            order by name, greeting
        ) as greeting
from mail_merge;
+---------+---------+--------------+
| country | name    | greeting     |
+---------+---------+--------------+
| Germany | Boris   | Guten morgen |
| Germany | Michael | Guten morgen |
| Sweden  | Bjorn   | Tja          |
| Sweden  | Mats    | Tja          |
| USA     | John    | Hello        |
| USA     | Pete    | Hello        |
+---------+---------+--------------+

LEAD()

The LEAD() method is used to calculate the value of rows after the current row.

Syntax:

Syntax

LEAD(expr[, offset[, default]])
OVER([<partition_by_clause>] [<order_by_clause>])

Parameters:

Calculate the next day's closing price and compare it to today's to see whether it is higher or lower.

select stock_symbol, closing_date, closing_price,
    case
        (lead(closing_price,1, 0)
            over (partition by stock_symbol
                  order by closing_date)
         - closing_price) > 0
    when true then "higher"
    when false then "flat or lower"
    end as "trending"
from stock_ticker
order by closing_date;
+--------------+---------------------+---------------+---------------+
| stock_symbol | closing_date        | closing_price | trending      |
+--------------+---------------------+---------------+---------------+
| JDR          | 2014-09-13 00:00:00 | 12.86         | higher        |
| JDR          | 2014-09-14 00:00:00 | 12.89         | higher        |
| JDR          | 2014-09-15 00:00:00 | 12.94         | flat or lower |
| JDR          | 2014-09-16 00:00:00 | 12.55         | higher        |
| JDR          | 2014-09-17 00:00:00 | 14.03         | higher        |
| JDR          | 2014-09-18 00:00:00 | 14.75         | flat or lower |
| JDR          | 2014-09-19 00:00:00 | 13.98         | flat or lower |
+--------------+---------------------+---------------+---------------+

MAX()

Returns the maximum value of the specified rows in the current window.

Syntax

MAX(expression) [OVER (analytic_clause)]

Example 8:

Calculate the maximum value of rows from the first row to the row after the current row.

select x, property,
    max(x)
        over (
            order by property, x
            rows between unbounded preceding and 1 following
        ) as 'local maximum'
from int_t
where property in ('prime','square');
+---+----------+---------------+
| x | property | local maximum |
+---+----------+---------------+
| 2 | prime    | 3             |
| 3 | prime    | 5             |
| 5 | prime    | 7             |
| 7 | prime    | 7             |
| 1 | square   | 7             |
| 4 | square   | 9             |
| 9 | square   | 9             |
+---+----------+---------------+

From StarRocks 2.4 onwards, you can specify the row range as rows between n preceding and n following, which means you can capture n rows before the current row and n rows after the current row.

Example statement:

select x, property,
    max(x)
        over (
            order by property, x
            rows between 3 preceding and 2 following) as 'local maximum'
from int_t
where property in ('prime','square');

MIN()

Returns the minimum value of the specified rows in the current window.

Syntax:

MIN(expression) [OVER (analytic_clause)]

Example 9:

Calculate the minimum value of rows from the first row to the row after the current row.

select x, property,
    min(x)
        over (
            order by property, x desc
            rows between unbounded preceding and 1 following
        ) as 'local minimum'
from int_t
where property in ('prime','square');
+---+----------+---------------+
| x | property | local minimum |
+---+----------+---------------+
| 7 | prime    | 5             |
| 5 | prime    | 3             |
| 3 | prime    | 2             |
| 2 | prime    | 2             |
| 9 | square   | 2             |
| 4 | square   | 1             |
| 1 | square   | 1             |
+---+----------+---------------+

From StarRocks 2.4 onwards, you can specify the row range as rows between n preceding and n following, which means you can capture n rows before the current row and n rows after the current row.

Example statement:

select x, property,
    min(x)
    over (
          order by property, x desc
          rows between 3 preceding and 2 following) as 'local minimum'
from int_t
where property in ('prime','square');

RANK()

The RANK() function is used to represent rankings. Unlike DENSE_RANK(), RANK() will appear as a vacant number. For example, if two tied 1s appear, the third number of RANK() will be 3 instead of 2.

Syntax:

RANK() OVER(partition_by_clause order_by_clause)

Example 10:

Ranking according to column x:

select x, y, rank() over(partition by x order by y) as rank
from int_t;
+---+---+------+
| x | y | rank |
+---+---+------+
| 1 | 1 | 1    |
| 1 | 2 | 2    |
| 1 | 2 | 2    |
| 2 | 1 | 1    |
| 2 | 2 | 2    |
| 2 | 3 | 3    |
| 3 | 1 | 1    |
| 3 | 1 | 1    |
| 3 | 2 | 3    |
+---+---+------+

ROW_NUMBER()

Returns a continuously increasing integer starting from 1 for each row of a Partition. Unlike RANK() and DENSE_RANK(), the value returned by ROW_NUMBER() does not repeat or have gaps and is continuously incremented.

Syntax:

ROW_NUMBER() OVER(partition_by_clause order_by_clause)

Example 11:

select x, y, row_number() over(partition by x order by y) as rank
from int_t;
+---+---+------+
| x | y | rank |
+---+---+------+
| 1 | 1 | 1    |
| 1 | 2 | 2    |
| 1 | 2 | 3    |
| 2 | 1 | 1    |
| 2 | 2 | 2    |
| 2 | 3 | 3    |
| 3 | 1 | 1    |
| 3 | 1 | 2    |
| 3 | 2 | 3    |
+---+---+------+

SUM()

Syntax:

SUM(expression) [OVER (analytic_clause)]

Example 12:

Group by property and calculate the sum of the current, preceding, and following rows within the group.

select x, property,
    sum(x)
        over (
            partition by property
            order by x
            rows between 1 preceding and 1 following
        ) as 'moving total'
from int_t where property in ('odd','even');
+----+----------+--------------+
| x  | property | moving total |
+----+----------+--------------+
| 2  | even     | 6            |
| 4  | even     | 12           |
| 6  | even     | 18           |
| 8  | even     | 24           |
| 10 | even     | 18           |
| 1  | odd      | 4            |
| 3  | odd      | 9            |
| 5  | odd      | 15           |
| 7  | odd      | 21           |
+----+----------+--------------+
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