The correct answer is C.

In the context of the Databricks Certified Data Engineer - Associate exam, understanding how different join types handle missing keys is a fundamental skill. Here is a breakdown of why Option C is the correct result of the LEFT JOIN operation.

Understanding the LEFT JOIN

A LEFT JOIN returns all records from the left table (sales), and the matched records from the right table (favorite_stores). If there is no match, the result is NULL on the side of the right table.

Step-by-Step Execution

1. Identify the Left Table: The table following FROM is sales. All rows from this table must appear in the final output.
   • a1 (present)
   • a3 (present)
   • a4 (present)
2. Match with Right Table: We look for corresponding customer_id values in favorite_stores.
   • a1: Found in favorite_stores. It matches with store_id s1.
   • a3: Not found in favorite_stores. Because this is a LEFT JOIN, the row is kept, and the store_id is filled with NULL.
   • a4: Found in favorite_stores. It matches with store_id s2.
3. Filter the Columns: The SELECT statement specifically asks for sales.customer_id, sales.spend, and favorite_stores.store_id.

Final Result Set

Why other options are incorrect:

• Option A & D: These include customer_id a2. This ID only exists in the right table (favorite_stores). In a LEFT JOIN, records that exist only in the right table are excluded. (These would only appear in a RIGHT JOIN or FULL OUTER JOIN).
• Option B: This represents an INNER JOIN. It excluded a3 because there was no matching record in the right table. A LEFT JOIN must include all records from the left table regardless of a match.