[Solved] Given the categories table:id | name | parent_id ---+------------+---------- 1 | Electronics| NULL 2 | Computers | 1 3 | Laptops | 2 4 | Phones | 1 What will the following query return?SELECT c.... | SQL

SQL - Advanced Joins

Given the categories table:

id | name       | parent_id
---+------------+----------
1  | Electronics| NULL
2  | Computers  | 1
3  | Laptops    | 2
4  | Phones     | 1

What will the following query return?

SELECT c.name AS Category, p.name AS Parent
FROM categories c LEFT JOIN categories p ON c.parent_id = p.id;

AA list of categories with their parent category names, NULL if no parent exists.

BOnly categories that have a parent category, excluding top-level categories.

CAll categories with their own names repeated as parent names.

DAn error because self join requires INNER JOIN, not LEFT JOIN.

Step-by-Step Solution

Solution:

Step 1: Understand the join type
LEFT JOIN returns all rows from c and matching rows from p, or NULL if no match.
Step 2: Join condition
Joining on c.parent_id = p.id links each category to its parent.
Step 3: Result interpretation
Top-level categories have parent_id NULL, so p.name will be NULL for them.
Final Answer:
A list of categories with their parent category names, NULL if no parent exists. is correct.
Quick Check:
LEFT JOIN preserves all categories including those without parents [OK]

Quick Trick: LEFT JOIN keeps all categories, parents NULL if none [OK]

Common Mistakes:

MISTAKES

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