Comparison Date Range (Previous Period) not working on Blended Data Source

Data Studio Data Studio Q&A
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Hi community,

I am trying to calculate a Month-over-Month (MoM) delta using the built-in “Comparison date range → Previous period” feature on a blended data source in Looker Studio, but it refuses to calculate the comparison data.

My Setup:

  • Data Source 1 (Left): HR Data from BigQuery (Dimensions: email, department).

  • Data Source 2 (Right): Telemetry Data from BigQuery (Dimensions: email, report_month; Metric: is_active).

  • Blend Configuration: Left Outer Join on the email field.

The Goal: I want to create a Table chart showing the aggregated is_active metric grouped by report_month, and display the MoM delta percentage using the built-in Comparison date range.

The Problem: When I apply the “Previous period” comparison to the chart based on the blended data, the delta percentage simply does not calculate/appear.

Troubleshooting Steps I’ve Already Taken:

  1. I ensured the report_month field is properly recognized as a Date. I even created a calculated field CAST(report_month AS DATE) to force the correct type.

  2. I set the format to “Year Month” (and tried “Date”).

  3. I explicitly added this date field to the “Date Range Dimension” slot in the right table of the Blend settings.

  4. I explicitly added it to the “Date Range Dimension” slot of the Chart settings.

  5. Running calculation (Running delta percentage) is not a viable workaround as it relies on table sorting and breaks when sorting chronologically.

My Question: Is this a known limitation of Data Blending in Looker Studio (losing the ability to perform period-over-period date math on aggregated joined data)? Are there any workarounds to make Looker Studio natively calculate the “Previous Period” on this blend, or is my only option to calculate the MoM delta using LAG() directly in BigQuery before connecting it to Looker Studio?

Thanks in advance for any insights!