You want to optimize a query joining employees, departments, and locations. Employees has 5000 rows, departments 50, locations 5. Which join order best improves performance?

hard📝 Application Q9 of 15

SQL - Advanced Joins

You want to optimize a query joining employees, departments, and locations. Employees has 5000 rows, departments 50, locations 5. Which join order best improves performance?

Query:

SELECT * FROM employees JOIN departments ON employees.dept_id = departments.id JOIN locations ON departments.loc_id = locations.id;

AJoin locations to employees first, then departments

BJoin employees to departments first, then locations

CJoin locations to departments first, then employees

DJoin departments to employees first, then locations

Step-by-Step Solution

Solution:

Step 1: Consider table sizes
Employees is largest, departments medium, locations smallest.
Step 2: Join large to medium first
Joining employees to departments first reduces rows early, then joining small locations last is efficient.
Final Answer:
Join employees to departments first, then locations -> Option B
Quick Check:
Large to medium, then small [OK]

Quick Trick: Join from largest to smallest tables for best performance [OK]

Common Mistakes:

MISTAKES

Joining smallest tables first causing inefficiency
Ignoring join condition correctness
Assuming join order does not affect performance

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More SQL Quizzes

You want to optimize a query joining employees, departments, and locations. Employees has 5000 rows, departments 50, locations 5. Which join order best improves performance?

Step 1: Consider table sizes

Step 2: Join large to medium first

Final Answer:

Quick Check:

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