Loading

Row Count Discrepancy with Null Join Keys in Tableau Prep

Publish Date: Jun 3, 2026
Description

When running a flow in Tableau Prep Builder or on Tableau Server, a discrepancy in record counts may occur between the “Summary of Join Results” displayed in the Prep Builder GUI and the actual output file when fields used in the join clause contain null values in both tables.

This discrepancy occurs because records containing null values in join fields may be evaluated differently during join processing. In SQL semantics, NULL = NULL is not considered TRUE; however, join evaluation behavior may vary depending on the execution context. As a result, the record counts shown in the “Summary of Join Results” and the actual output may differ when join keys contain null values.

Resolution

Option 1: Replace NULL values with standardized values.

  1. In the clean steps prior to the join step, create a calculated field using IFNULL() or ZN() to replace null values with a standardized string or numeric value (for example, "N/A" or 0).
  2. Update the join clauses to use the new calculated field instead of the original field containing null values.

Option 2: Remove the NULL-only field from the join condition.

  1. If the field containing null values is not required as a join key, remove that field from the join clauses.
  2. Verify that the remaining join keys still produce the intended relationship between the two tables.
Additional Resources

As an example of how NULL evaluates in Microsoft Transact-SQL: NULL and UNKNOWN (Transact-SQL)

Knowledge Article Number

005385506

 
Loading
Salesforce Help | Article