hardMultiple Choice
FC0-U71 Practice Question: A query uses a LEFT JOIN between Customers and…
A query uses a LEFT JOIN between Customers and Orders. The result set includes customers with no orders. What does the Orders columns show for those customers?
⚠ Common exam trap
Candidates often confuse NULL with zero or an empty string, thinking a missing record should show a default value like 0 or '', but SQL strictly uses NULL to represent the absence of a matching row in a LEFT JOIN.
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
✓
NULL
In a LEFT JOIN, all rows from the left table (Customers) are retained, and for rows with no matching rows in the right table (Orders), the columns from the right table are filled with NULL. This is the standard SQL behavior defined by the SQL standard (ISO/IEC 9075). Therefore, for customers with no orders, the Orders columns show NULL, not zero, an empty string, or any other default value.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
NULL
Why this is correct
A LEFT JOIN preserves every row from the left table (Customers) and matches rows from Orders where possible. When no matching order exists, the unmatched right-side columns are filled with NULL, which is exactly the behaviour the stem describes.
- ✗
The same as the customer ID
Why it's wrong here
A LEFT JOIN generates NULLs for the unmatched side rather than copying values across tables. Repeating the customer ID would require an explicit COALESCE or ISNULL expression in the SELECT list, which the query as written does not include.
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Zero
Why it's wrong here
NULL represents the absence of a value, not the numeric value zero. Zero is a stored number requiring a matching Orders row or an explicit COALESCE replacement; the join itself returns NULL for customers with no orders.
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Empty string
Why it's wrong here
Unmatched rows produce NULL in every Orders column, because no row exists to supply a value. An empty string is a real stored value of zero length, which SQL never fabricates for a missing join match; it would only appear if the column itself contained one.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This FC0-U71 practice question is part of Courseiva's free CompTIA 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 FC0-U71 exam.