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Cover image for Why you need SQL window functions (part 2)

Why you need SQL window functions (part 2)

Helen Anderson
Data wrangler, AWS Data Hero, and tag moderator on Dev.to.
Originally published at helenanderson.co.nz Updated on ・3 min read

Welcome back to the world of window functions.

Window functions are closely related to aggregate functions. But rather than collapsing all the rows we want into one row, we keep them and add a new column with a running total, rank or moving average. This becomes our ‘window frame.’

It becomes clearer with examples and pictures, so if you’re interested in using this powerful tool, keep reading.


1 – Introduction
2 – Setting the scene
3 – Bucket rows into groups
4 – Access a total from an earlier row
5 – Use Window Functions to perform calculations


Introduction

Window functions come in three main types. These are:

Aggregate Window Functions

These use aggregate functions like SUM, COUNT, MAX, MIN over a set of rows and return a single result from the query.

Ranking Window Functions

These assign a ‘rank’ to a set of rows and using RANK, DENSE_RANK, ROW_NUMBER, NTILE

Value Window Functions

These use LAG, LEAD, FIRST_VALUE, LAST_VALUE to access a previous row without having to do a self-join.


Setting the scene

This post is picking up where we left off in Part One, where we used window functions to answer questions from a Sales Manager who is setting targets for the year.

In the last post we covered aggregate window functions and how to use RANK(). In this post, we’ll be exploring how to use NTILE(), then exploring how we can use value window functions to answer questions using sales data.


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Bucket rows into groups

When we left her in the last post we had provided the Sales Manager two tables. One showing daily sales with a running total, and all sales ranked by their dollar value.

This time the Sales Manager would like to give a bonus to the territory who has bought in the top 25% of customers over the last year.

To do this we can use NTILE() to bucket the dollar values into four groups that represent four quartiles.

select
  territoryid,
  customerid,
  sum(subtotal) as subtotal,
  ntile(4) over(order by sum(subtotal)) as bucket
from 
  sales.salesorderheader 
where sales_date between 
'2018-01-01' and '2019-01-01'
group by 
  territoryid, 
  customerid
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NTILE() can be used to bucket groups of rows and can be adjusted by changing the number in the brackets.


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Access a total from an earlier row

The Sales Manager needs one last thing before the targets for next year can be locked in. A table that shows the ‘big customers’ identified earlier and the dates they are making orders.

What the Sales Manager needs for this last piece of work is an idea of how many days there are between orders for these big customers. If she can anticipate when they are going to buy next we can get in touch with them earlier.

select
  customerid, 
  subtotal, 
  sale_date,
  lag(sale_date) over(order by sale_date) as last_order,
from sales.salesorderheader 
where 
  customerid = 11078
  and sale_date between 
'2018-01-01' and '2019-01-01'
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By using LAG() we are able to reach up and return the result from the previous row and add it to a new column. LEAD() works in the same way but in reverse.


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Use Window Functions to perform calculations

To make this useful for the Sales Manager we can use CAST and a new column to calculate the difference between the dates.

select
  customerid, 
  subtotal, 
  sale_date,
  lag(sale_date) over(order by sale_date) as last_order,
  cast(sale_date - (lag(sale_date) 
over(order by sale_date)) as int) as days_between
from sales.salesorderheader 
where 
  customerid = 11078
  and sale_date between '2018-01-01' and '2019-01-01'
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I hope you’ve enjoyed these two posts on SQL window functions. How do you use these in your work? Do you find them a useful and powerful tool?


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This post originally appeared on helenanderson.co.nz

Discussion (3)

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ryands17 profile image
Ryan Dsouza

The best part of ntile being I use it a lot to provide custom names to the rankings. For e.g. giving names like HIGH, AVERAGE and LOW based on the groupings done by ntile. A combination of case, ntile and sum covers a lot of use-cases.

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aarone4 profile image
Aaron Reese

One issue with window functions is that you can't use the resulting value in the WHErE subclause or as a JOIN predicate. However you can get around this by storing the original query in a common table expression and then you CAN use the value as it is now encapsulated in the driving dataset

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helenanders26 profile image
Helen Anderson Author

Thanks Aaron, that's a great tip :D