PL-300 Prepare the data Practice Question
You are merging two tables in Power Query: Orders and Customers. The Orders table has a CustomerID column, and the Customers table has a CustomerID column. You want to keep all rows from Orders and only matching rows from Customers. Which join kind should you use?
⚠ Common exam trap
Test-takers frequently confuse Left Outer with Right Outer, mistakenly thinking that 'keeping all rows from Orders' means the Orders table should be on the right side, when in Power Query the join direction is determined by which table is selected as the primary table in the Merge dialog.
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
✓
Left Outer
(Left Outer) is correct because it returns all rows from the Orders table regardless of whether a matching CustomerID exists in the Customers table. When no match is found, the Customers columns will contain null values. This is the standard behavior for a left outer join in Power Query, which preserves the left table's rows entirely.
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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Full Outer
Why it's wrong here
Full outer join in Power Query returns every row from both the orders table and the customers table, matching where possible and inserting nulls for non-matches. While it preserves all data, it also introduces customer rows that have no corresponding order, which is usually not desired when the goal is to keep all orders and only enrich them with customer details. This makes it incorrect because it bloats the result with irrelevant customer-only records and can complicate downstream analysis.
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Inner
Why it's wrong here
Inner join in Power Query keeps only rows where a matching key exists in both the orders and customers tables. It effectively discards any order that lacks a matching customer record, which can silently drop business data. If the requirement is to preserve every order row regardless of whether customer information exists, an inner join is too restrictive and therefore wrong for this merge scenario.
- ✓
Left Outer
Why this is correct
Left outer join in Power Query preserves every row from the left table (orders) and adds columns from the right table (customers) only where a match exists, filling non-matches with null. This guarantees that the order count remains unchanged, which is typically the primary business requirement when merging order records with customer reference data. It is the correct choice because it retains all orders while still enriching them with available customer attributes.
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Right Outer
Why it's wrong here
Right outer join in Power Query keeps all rows from the right table (customers) and only brings in matching rows from the left table (orders). This reverses the intended direction, causing any order without a matching customer to be dropped and including customers who have never placed an order. Since the task is to analyze orders, losing order rows is unacceptable, making right outer join incorrect for this scenario.
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