DP-203 Develop data processing Practice Question
You are implementing a mapping data flow in Azure Data Factory that processes data from an Azure SQL Database. The data flow includes a derived column transformation that adds a new column based on a complex expression. You need to ensure the expression handles null values appropriately. Which function should you use to replace null values with a default?
⚠ Common exam trap
Watch out — candidates often confuse functions that check for nulls with those that replace them, or using a more complex conditional expression when a dedicated function exists.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
coalesce
The coalesce function is designed to return the first non-null value from a list, making it ideal for replacing nulls with a default. In a derived column transformation, you can write coalesce(column, 'default') to substitute nulls. Other functions like isNull, nullIf, and iif serve different purposes: isNull checks for null, nullIf creates null, and iif requires a condition. Therefore, coalesce is the correct function for this scenario.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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nullIf
Why it's wrong here
The nullIf function returns null if two expressions are equal, otherwise returns the first expression. It is used to create nulls, not to replace them. For example, nullIf(column, '') would return null if the column is an empty string. This is the opposite of what is needed. Therefore, it does not help in replacing nulls with a default value.
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iif
Why it's wrong here
The iif function evaluates a condition and returns one of two values. While you can use iif(isNull(column), default, column) to replace nulls, it requires an additional isNull check and is more complex. The coalesce function is designed specifically for this purpose and is simpler. Thus, iif is not the best choice for replacing nulls directly.
- ✓
coalesce
Why this is correct
The coalesce function returns the first non-null value from a list of expressions. In a derived column transformation, you can use coalesce(column, default) to replace nulls with a default value. This is the standard way to handle nulls in Azure Data Factory mapping data flows. It is efficient and works with various data types, ensuring that null values are substituted appropriately.
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isNull
Why it's wrong here
The isNull function checks whether a value is null and returns a boolean. It does not replace null values; it only identifies them. You could use it in a conditional expression like iif(isNull(column), default, column), but that is more verbose. The question asks for a function to replace null values, so isNull alone is not sufficient. It is used for testing, not substitution.
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Last reviewed September 2026 · checked against the official Microsoft exam blueprint
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