BigData / Tableau Interview Questions
How does Tableau handle NULL values?
Tableau treats NULL as "no value," and handles it differently depending on where it shows up: an axis, a dimension header, or inside a calculation.
- On a continuous axis (like a date or number line): Tableau shows an indicator icon in the bottom-right of the view noting that some marks are not shown due to NULLs, since a NULL can't be plotted on a numeric axis.
- As a dimension member: NULL values typically appear grouped together under a "Null" header, treated like any other category value.
- In calculations: functions like
ISNULL()andIFNULL()let you explicitly detect or substitute NULLs; arithmetic involving a NULL generally propagates to a NULL result unless handled.
IFNULL([Discount], 0) -- substitutes 0 wherever Discount is NULL ISNULL([Ship Date]) -- returns TRUE/FALSE
A common gotcha: a NULL on an axis silently hides affected marks rather than erroring out, which is why checking for that small NULL indicator icon is a good habit whenever a chart looks like it's missing data points you expected to see.
What happens when a NULL value falls on a continuous axis in a Tableau view?
Which function substitutes a specified value wherever a field is NULL?
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