Friday, February 19, 2021

Main SQL Window Functions for Data Engineers in Cloud

 To become a data engineer in cloud requires to have a good grasp of SQL among various other things. SQL is the premier tool for interacting with data sets. At first it seems daunting to see all those SQL analytics functions, but if you start with a tiny dataset like in the examples below and understand how these functions work, then it all becomes very easy for large datasets of any volume.

Once you know the basic structure of SQL, understand the basic clauses, then its time to jump into the main analytics functions. Below I have used SQL's With clause to generate a tiny dataset in Oracle. You don't have to create a table, load it with sample data and play with it. Just use with clause with the accompanying select statements which demonstrate you the common SQL Window functions.


1- In this example, sum and row_number functions works on each row of whole window.

   

With x as ( 

   SELECT 'tom' as name, 1 AS t from dual

   UNION ALL

   SELECT 'harry' as name,2 AS t  from dual

   UNION ALL

   SELECT 'jade' as name,2 AS t  from dual

   UNION ALL

   SELECT 'ponzi' as name,3 AS t  from dual

)

select name,t,sum(t) over () as SumEachRow, row_number() over (order by t) as RN from x;


2- In this example, sum and row_number functions works on each row of each partition of whole window. This window is partitioned on column t.


With x as ( 

   SELECT 'tom' as name, 1 AS t from dual

   UNION ALL

   SELECT 'harry' as name,2 AS t  from dual

   UNION ALL

   SELECT 'jade' as name,2 AS t  from dual

   UNION ALL

   SELECT 'ponzi' as name,3 AS t  from dual

)

select name,t,sum(t) over (partition by t) as SumEachRow, row_number() over (partition by t order by t) as RN from x;


3- In following example, we have divided the window into 2 partitions by using case statement within partition clause. One partition is when t=1, and other partition is composed of rest of rows.


With x as ( 

   SELECT 'tom' as name, 1 AS t from dual

   UNION ALL

   SELECT 'harry' as name,2 AS t  from dual

   UNION ALL

   SELECT 'jade' as name,2 AS t  from dual

   UNION ALL

   SELECT 'ponzi' as name,3 AS t  from dual

)

select name,t,sum(t) over (partition by CASE WHEN t = 1 THEN t ELSE NULL END) as SumEachRow, row_number() over (partition by CASE WHEN t = 1 THEN t ELSE NULL END order by t) as RN from x;


4- Below example is variant of example 3. In this the window function row_number is working on whole window instead of partition whereas the window function sum is working on partitions.


With x as ( 

   SELECT 'tom' as name, 1 AS t from dual

   UNION ALL

   SELECT 'harry' as name,2 AS t  from dual

   UNION ALL

   SELECT 'jade' as name,2 AS t  from dual

   UNION ALL

   SELECT 'ponzi' as name,3 AS t  from dual

)

select name,t,sum(t) over (partition by CASE WHEN t = 1 THEN t ELSE NULL END) as SumEachRow, row_number() over (order by t) as RN from x;


5- This example uses lag function to return previous value of window function. For lag function, the value for first row is always null as there is no previous value.


With x as ( 

   SELECT 'tom' as name, 1 AS t from dual

   UNION ALL

   SELECT 'harry' as name,2 AS t  from dual

   UNION ALL

   SELECT 'jade' as name,2 AS t  from dual

   UNION ALL

   SELECT 'ponzi' as name,3 AS t  from dual

)

select name,t,lag(t) over (order by t) as Previous_t from x;


6- This example uses lead function to return next value of window function. For lead function, the value of last row is always null as there is no next value.


With x as ( 

   SELECT 'tom' as name, 1 AS t from dual

   UNION ALL

   SELECT 'harry' as name,2 AS t  from dual

   UNION ALL

   SELECT 'jade' as name,2 AS t  from dual

   UNION ALL

   SELECT 'ponzi' as name,3 AS t  from dual

)

select name,t,lead(t) over (order by t) as Next_t from x;


7- This example shows that First_value function returns first value in window for each row.


With x as ( 

   SELECT 'tom' as name, 1 AS t from dual

   UNION ALL

   SELECT 'harry' as name,2 AS t  from dual

   UNION ALL

   SELECT 'jade' as name,2 AS t  from dual

   UNION ALL

   SELECT 'ponzi' as name,3 AS t  from dual

)

select name,t,first_value(t) over (order by t) as First_t from x;


8- This example shows that First_value function returns first value in each partition of window for each row.

With x as ( 

   SELECT 'tom' as name, 1 AS t from dual

   UNION ALL

   SELECT 'harry' as name,2 AS t  from dual

   UNION ALL

   SELECT 'jade' as name,2 AS t  from dual

   UNION ALL

   SELECT 'ponzi' as name,3 AS t  from dual

)

select name,t,first_value(t) over (partition by t order by t) as First_t from x;


9- This example shows that last_value function returns last value in window for each row.


With x as ( 

   SELECT 'tom' as name, 1 AS t from dual

   UNION ALL

   SELECT 'harry' as name,2 AS t  from dual

   UNION ALL

   SELECT 'jade' as name,2 AS t  from dual

   UNION ALL

   SELECT 'ponzi' as name,3 AS t  from dual

)

select name,t,last_value(t) over (order by t ROWS BETWEEN

           UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) as Last_t from x;


10- This example shows that Last_value function returns last value in each partition of window for each row.

With x as ( 

   SELECT 'tom' as name, 1 AS t from dual

   UNION ALL

   SELECT 'harry' as name,2 AS t  from dual

   UNION ALL

   SELECT 'jade' as name,2 AS t  from dual

   UNION ALL

   SELECT 'ponzi' as name,3 AS t  from dual

)

select name,t,last_value(t) over (partition by t order by t ROWS BETWEEN

           UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) as Last_t from x;


For explanation of rows between unbounded clause, see this 

11- This example shows the rank() function which is useful for Top N, or Bottom N sort of queries. Following is for whole window. The main idea is that rank starts from 1 from first row and then rank remains same for rows with same value within window. When value changes, the rank increments as per number of lines from top. 

With x as ( 

   SELECT 'tom' as name, 1 AS t from dual

   UNION ALL

   SELECT 'harry' as name,2 AS t  from dual

   UNION ALL

   SELECT 'jade' as name,2 AS t  from dual

   UNION ALL

   SELECT 'ponzi' as name,3 AS t  from dual

)

select name,t,rank(t) over (order by t) as Rank from x;


12- This example shows the rank() function which is useful for Top N, or Bottom N sort of queries. Following is for each partition of window.


With x as ( 

   SELECT 'tom' as name, 1 AS t from dual

   UNION ALL

   SELECT 'harry' as name,2 AS t  from dual

   UNION ALL

   SELECT 'jade' as name,2 AS t  from dual

   UNION ALL

   SELECT 'ponzi' as name,3 AS t  from dual

)

select name,t,rank() over (partition by t order by t) as Rank from x;


PS. Yes I know formatting of code chunks is not good enough but this is limitation of blogger platform it seems and another note to self that I need to move to a better one.

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