Some questions require a virtual table inside a measure to create the required data structure.
Requirement
How many customers have purchased a product multiple times?
Several products have been purchased multiple times but we want to know how many customers have purchased each product more than once.
Set-up
This Power BI data model has three tables:
- Transaction
- Customer
- Product
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Below we see some of the 27 transactions.
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Below we see the final viz. ‘Product Name’ is the filter context (no other filters or slicers) and my measure (I couldn’t think of a shorter name).
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1 person (Zack) made 2 separate purchases of Lime. 2 people (Shawn & Deonna) each made 2 separate Mango purchases. No one else bought the same fruit more than once.
In the Total area the filter context for ‘Product Name’ is empty but I included ‘Product Name’ inside SUMMARIZE so we get 3. Had I not included ‘Product Name’ then the total would have returned 5!?
WHY? In the total row the filter context for ‘Product Name’ would be empty and anyone with >1 purchase (same or different fruit) would count.
Solution
DAX code
Customers Who Purchased Same Prod Multi Times = COUNTROWS( FILTER( SUMMARIZE('Transaction', 'Product'[Product Name], Customer[Customer ID],"All Purchases", COUNTROWS('Transaction') ), [All Purchases]>=2 ) )
DAX details
Starting from the inside and going outwards:
- SUMMARIZE creates aggregated data. Grouping by Product Name and CustomerID and then counting their transactions
- Wrap it with FILTER to exclude product/customer combinations that have only 1 purchase
- For each ‘Product Name’ COUNTROWS counts remaining rows
Key: SUMMARIZE function does a group by and counts.
Why? Transaction table is based on individual transactions. We need to determine the total transaction count for each product/customer combination and only then filter out some rows.
The relationship between one side tables (Customer & Product) and many side table (Transaction) lets us to use all tables inside SUMMARIZE function.
See Virtual Table Rows
What rows are found in the virtual SUMMARIZE table? Let’s create a physical table to take a look. This is not a best practice because in a real model it could greatly increase the pbix file size.
On the ribbon click ‘Modeling’, ‘New Table’ and paste in DAX below:
test = SUMMARIZE('Transaction','Product'[Product Name],Customer[Customer ID],"All Purchases", COUNTROWS('Transaction'))
Below I sorted by column ‘All Purchases’. 2 customers have 2 different Mango purchases and 1 person has 2 different Lime purchases.
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DAX Syntax Links
COUNTROWS https://dax.guide/countrows/
SUMMARIZE https://dax.guide/summarize/
FILTER https://dax.guide/filter/
About Me
This is my personal blog to practice learning Power BI (Power Query, DAX, etc) and share my knowledge with you.