Understanding Amazon Redshift Window Functions: Made Easy 101

There is a surge in data generation by organizations, and this growth has been exponential, therefore, giving rise to technologies and pathways in which this data could be harnessed for proper analysis.

There are lots of available tools to handle Big Data and most of them are built on the fundamentals of SQL making it easy to manipulate and use. Amazon Redshift is one of such data housing platforms that allows you to arrange, structure, and manage your data effectively using its various facilities for efficient analytics with useful functions such as Amazon Redshift Window Functions.

Amazon Redshift Window Functions can be used in computing an aggregate value that is based on a group of rows in a table to save a lot of time during development, and these will be discussed in full in this article showcasing its syntax and mentioning some of the Amazon Redshift Window Functions available.

Introduction to Amazon Redshift

Amazon Redshift is a Cloud Data Warehouse Analytics system that helps you to analyze all your data. Amazon Redshift makes it easy for you to gain new insights as you can query and combine exabytes of structured and semi-structured data from the Data Warehouse, Operational Database, and Data Lake using standard SQL. 

Amazon Redshift offers good price performance compared to other Cloud Data Warehouses. Moreover, it provides the best price-performance at any scale by using AWS designed-hardware and Machine Learning (ML) services.

Introduction to Amazon Redshift Window Functions

A Window Function in SQL, also known as the Analytical Function, can be defined as a function that uses values from one or multiple rows to return a value for each row and has an OVER clause. It is the opposite of the aggregate Function, which returns a single value for multiple rows and does not have an OVER clause. 

The window function in Amazon Redshift allows database developers to carry out analysis over partitions of information swiftly. Before the introduction of the Window Function, you will need to create sub-queries or common table expressions that will allow the creation of Windows, where primary queries will then work over the sub-queries to produce a result.

By using Window Functions, users are enabled to create Analytic business queries by operating on a partition called the Window of a result set. It then returns a value for every row in that Window and unlike the Group Function that aggregates result rows, all rows in a table expression are retained in a Window Function.

The values returned are calculated based on values from the sets of rows in that Window and for each row, the set of rows that is used to compute additional attributes is defined by the window. 

The OVER clause is a Window specification that is used in defining a window, and it is based on the following concepts:

• Windows Partitioning (PARTITION clause): This will form groups of rows.
• Windows Ordering (ORDER BY clause): This defines an order of sequence of rows within each partition.
• Windows Frames (ROWS specification): This is defined relative to each row to further restrict the set of rows.

Moreover, Window Functions are the last set of operations performed in a query apart from the final ORDER BY clause as all joins and all WHERE, GROUP BY, and HAVING clauses operations are carried out before the Windows Functions. Therefore, Window Functions appear only in the select list or ORDER BY clause.

Finally, multiple window functions can be used within a single query with different frame clauses, and they can also be used in other scalar expressions like CASE.

There are two types of Window Functions supported in Amazon Redshift, namely Aggregate and Ranking, which are further broken down to include the following groups:

Some of the supported Aggregate Functions are:

Aggregate Functions	Descriptions
AVG	Calculates the average value of a numeric column.
COUNT	Counts the number of rows or non-NULL values.
CUME_DIST	Calculates the cumulative distribution of values within a partition.
FIRST_VALUE	Returns the first value in a sorted partition of a set of values.
LAG	Accesses data from the previous row within a partition.
LAST_VALUE	Returns the last value in a sorted partition of a set of values.
LEAD	Accesses data from the following row within a partition.
MAX	Returns the maximum value in a set of values.
MEDIAN	Calculates the middle value of a sorted set of values.
MIN	Returns the minimum value in a set of values.
NTH_VALUE	Returns the nth value in a sorted set of values.
PERCENTILE_CONT	Computes a continuous percentile value in a set of data
PERCENTILE_DISC	Computes a discrete percentile value in a set of data.
RATIO_TO_REPORT	Computes the ratio of a value to the sum of values in a partition.
STDDEV_POP	Calculates the population standard deviation of a set of values.
STDDEV_SAMP (synonym for STDDEV)	Computes the sample standard deviation of a set of values.
SUM	Returns the sum of a set of values.
VAR_POP	Calculates the population variance of a set of values.
VAR_SAMP (synonym for VARIANCE)	Computes the sample variance of a set of values.

Some of the supported Ranking Functions are:

Ranking Functions	Description
DENSE_RANK	Assigns a unique rank to each row within a partition, without gaps in ranking values.
NTILE	Divide rows into a specified number of equal buckets.
PERCENT_RANK	Calculates the relative rank of a row as a percentage.
RANK	Assign a rank to each row, with gaps in ranking values if there are ties.
ROW_NUMBER	Assigns a sequential integer to each row in a partition.

Understanding Amazon Redshift Window Functions Syntax

Standard Amazon Redshift Window Functions have the following syntax. The following arguments are described subsequently.

function (expression) OVER (
[ PARTITION BY expr_list ]
[ ORDER BY order_list [ frame_clause ] ] )

Arguments

• function: This describes the individual function.
• OVER: This clause is used to define the Window specification and it is mandatory for Window Functions as it differentiates Window Functions from other SQL functions.
• PARTITION BY expr_list: This clause subdivides the result set into partitions. If a partition clause is present in a query, the function is calculated for the rows in each partition and if no partition clause is specified then the Function is computed for that single partition that contains the entire table. This clause is therefore optional.
• ORDER BY order_list: This Window Function is applied to rows within each partition sorted according to the order specification in ORDER BY. The ORDER BY clause can be used without the PARTITION BY clause and it is optional. For Ranking Functions, the ORDER BY clause identifies the measures for the ranking values. On the other hand, you need to order your partition rows before computing each frame using the Aggregate Function.
• column_name: This is the name of the column to be partitioned by or ordered by.
• ASC | DESC: This option defines the sort order for the expression in either ascending if no option is specified by default or descending orders when specified. For ASC the sort order can be from low to high for numeric values and A to Z for strings, while in DSC, it will be from high to low for numeric values and Z to A for strings.
• NULLS FIRST | NULLS LAST: This option is used to specify whether NULLS should be ordered first before non-null values or, last after non-null values. By default, NULLS are sorted and ranked last in ASC ordering and sorted and ranked first in DESC order.
• frame_clause: The frame clause is used to refine the set of rows in a Function’s Window when using ORDER BY for Aggregate Functions as it does not apply to Ranking Functions. It enables you to include or exclude sets of rows within the ordered result.
• ROWS: This clause defines the Window frame by specifying a physical offset from the current flow. It specifies the rows in the current Window that the value in the current row is to be combined with.

Examples of Amazon Redshift Window Functions

For clarity, you will be introduced to some examples of Amazon Redshift Window Functions and their corresponding syntax. These will include the following:

• Amazon Redshift Window Functions: SUM
• Amazon Redshift Window Functions: COUNT
• Amazon Redshift Window Functions: RANK
• Amazon Redshift Window Functions: FIRST_VALUE and LAST_VALUE

1) Amazon Redshift Window Functions: SUM

The SUM Window Function works on numeric values and ignores NULL values. It returns the sum of the input column or expression values. The SUM syntax is shown below.

SUM ( [ ALL ] expression ) OVER
(
[ PARTITION BY expr_list ]
[ ORDER BY order_list 
                        frame_clause ]
)

2) Amazon Redshift Window Functions: COUNT

The COUNT Window Function is used to count the rows defined by the expression. The COUNT Function has two variations, COUNT (*) and COUNT (expression).

• COUNT (*) is used to count all the rows in the target table whether they include nulls or not.
• COUNT (expression) computes the number of rows with non-NULL values in a specific column or expression.

The syntax for COUNT Function is displayed below:

COUNT ( * | [ ALL ] expression) OVER
(
[ PARTITION BY expr_list ]
[ ORDER BY order_list 
                        frame_clause ]
)

3) Amazon Redshift Window Functions: RANK

The RANK Window Function id is used to determine the rank of a value in a group of values based on the ORDER BY expression in the OVER clause. If the optional PARTITION BY clause is present then for each group of rows, rankings are reset.

RANK () OVER
(
[ PARTITION BY expr_list ]
[ ORDER BY order_list ]
)

4) Amazon Redshift Window Functions: FIRST_VALUE and LAST_VALUE

FIRST_VALUE returns the value of the specified expression for the first row in the Window frame. The LAST_VALUE function returns the value of the expression for the last row in the frame.

FIRST_VALUE | LAST_VALUE
( expression [ IGNORE NULLS | RESPECT NULLS ] ) OVER
(
[ PARTITION BY expr_list ]
[ ORDER BY order_list frame_clause ]
)

Want to master the use of Redshift FIRST_VALUE? Explore our guide to understand how to leverage this function for retrieving the first value in your data queries.

Conclusion

This article has given you a basic understanding of Amazon Redshift Window Functions by explaining what it is, its usefulness, and syntax, alongside various examples. Using Amazon Redshift Window Functions in your Database can help you perform operations speedily and come up with creditable analysis, but it can also be difficult as you will need to know the syntax to use for each Function which can be confusing at times; however, relying on a platform like Hevo Data can make the process seamless.

Hevo Data is a cloud data solution that helps arrange, group, and structure your data while integrating with cloud data warehouses like Amazon Redshift.

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