In this post, I discuss two of the most common problems when creating blends in Looker Studio.
If you want to read more about what blends or blended data sources are in Looker Studio, visit the post dedicated to this topic.
1. Duplicate Rows
You may have noticed that at the end of each example in the post about blends in Looker Studio, I stated the minimum number of rows that the combined table should have. There is a common misconception that the result of a left join should have the same number of rows as the left table, and a right join should match the right table. This is not the case, and you will see why in the following example.
For instance, I have a table listing customers along with the product they purchased, the quantity, and the purchase date.

On the other hand, I have another table with the product prices.

I want to calculate the total sales, and for that, I need to create a combined table where I can see both the quantity and the price to multiply them. To do this, I perform a left join of the customer table on the left with the prices table on the right, and this is the result.

Do you notice anything unusual? Have you realised that the customer ID LS-17200, who purchased a 3M Office Air Cleaner, appears repeatedly?
This is because the price list had two different prices for the 3M Office Air Cleaner. The field I used for the left join was only the product name, and since this is repeated in the right table, it will also repeat in the resulting table.
To resolve this, I need to use two join fields: product name and purchase date. This way, the join will know that the price for the product purchased by customer LS-172000 was the one on 1 August 2014.
You must be cautious when creating joins and expecting some columns to have unique values. If they do not, this can lead to ending up with duplicate values in the resulting table, as seen in the example.
2. Performance
If you combine very large datasets, they can impact the performance of your report. Sometimes, very long datasets may cause errors in Looker Studio and prevent graphs from loading, so it’s better to avoid making blends with a large number of dimensions.
There are more issues that can arise when creating blends in Looker Studio, but these are two of the most important to keep in mind. Especially the first one, as a poorly constructed join will display incorrect data.
If you have any questions, feel free to leave a comment! 🙂