Databricks-Spark-Assoc Using Spark SQL Practice Question
What is the result of applying the COALESCE function in Spark SQL when multiple arguments are provided?
⚠ Common exam trap
Candidates often confuse COALESCE with ISNULL or NVL, mistakenly believing it returns the first null value encountered or performs a conditional check rather than returning the first non-null argument found.
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
✓
It returns the first non-null argument.
The COALESCE function is a standard SQL tool for handling null values. It returns the first non-null argument from a list. This is extremely useful in data engineering for providing default values or merging columns that have sparse data. Understanding how to use it helps in writing cleaner SQL that handles missing values without needing complex CASE WHEN statements, which improves code readability and maintainability.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
It returns the sum of all arguments.
Why it's wrong here
The COALESCE function is not an aggregation function. It is a conditional selection function. Summing values is the job of the SUM() function. Using COALESCE for numeric addition will result in unexpected behavior, as it only returns a single value from the list, not a mathematical total of inputs.
- ✓
It returns the first non-null argument.
Why this is correct
COALESCE iterates through its arguments in order and returns the first one that is not null. If all arguments are null, it returns null. This is the idiomatic way in Spark SQL to perform null-replacement or provide fallback values for columns containing missing or null data in source systems.
- ✗
It returns the last non-null argument.
Why it's wrong here
The logic of COALESCE is to short-circuit upon finding the first valid value. The last argument is only returned if it is the only one not null, or if all preceding arguments are null. It does not scan the entire list to find the 'last' one, making this an incorrect description of operation.
- ✗
It throws an error if any argument is null.
Why it's wrong here
Handling null values is the primary purpose of COALESCE. It is designed to safely ignore nulls until a non-null value is encountered. It would be counter-productive if the function threw an error when encountering nulls, as that would invalidate the entire reason for its existence in SQL query logic.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Databricks exam blueprint
This Databricks-Spark-Assoc practice question is part of Courseiva's free Databricks certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the Databricks-Spark-Assoc exam.