[Solved] What is the logical issue in this query?SELECT e.name, d.dept_name FROM employees e LEFT JOIN departments d ON e.dept_id = d.id LEFT JOIN projects p ON p.dept_id = d.id WHERE p.project_name = 'Alpha'; | SQL

SQL - LEFT and RIGHT JOIN

What is the logical issue in this query?

SELECT e.name, d.dept_name FROM employees e LEFT JOIN departments d ON e.dept_id = d.id LEFT JOIN projects p ON p.dept_id = d.id WHERE p.project_name = 'Alpha';

AThe WHERE clause filters out rows where projects are NULL, negating the LEFT JOIN effect.

BThe JOIN condition between employees and departments is incorrect.

CThe query should use INNER JOIN instead of LEFT JOIN for departments.

DThe SELECT clause is missing the project name.

Step-by-Step Solution

Solution:

Step 1: Understand LEFT JOIN with WHERE
LEFT JOIN preserves all left table rows, but WHERE filters after joins.
Step 2: Analyze WHERE clause effect
Filtering on p.project_name = 'Alpha' excludes rows where p is NULL, effectively converting LEFT JOIN to INNER JOIN.
Final Answer:
The WHERE clause filters out rows where projects are NULL, negating the LEFT JOIN effect. -> Option A
Quick Check:
WHERE on right table columns can nullify LEFT JOIN [OK]

Quick Trick: WHERE on right table columns removes unmatched rows [OK]

Common Mistakes:

MISTAKES

Assuming LEFT JOIN always preserves all left rows
Placing filtering conditions on right table columns in WHERE
Confusing JOIN conditions with filtering

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More SQL Quizzes

What is the logical issue in this query?

Step 1: Understand LEFT JOIN with WHERE

Step 2: Analyze WHERE clause effect

Final Answer:

Quick Check:

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