Beginner LevelSQL & MySQL
What are INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN?
❓ Interview Prompt: Explain relational JOIN types, Venn diagram representation, matching key criteria, and handling missing NULL fields.
Relational JOINs combine columns from one or more tables based on matching key relationships:
4 Core Join Types:
1
INNER JOIN: Returns rows ONLY when there is a matching key in BOTH tables.
2
LEFT (OUTER) JOIN: Returns ALL rows from the left table, plus matching rows from the right table. Unmatched right table columns return 'NULL'.
3
RIGHT (OUTER) JOIN: Returns ALL rows from the right table, plus matching rows from the left table.
4
FULL (OUTER) JOIN: Returns all rows when there is a match in EITHER left or right table.
Code Implementation Example
sql
-- INNER JOIN vs LEFT JOIN example
SELECT u.id, u.name, o.order_date, o.total_amount
FROM users u
LEFT JOIN orders o ON u.id = o.user_id
WHERE u.status = 'ACTIVE';💡Joining user and order tables with a LEFT JOIN.
🎯 Interview Key Takeaway
Use INNER JOIN to fetch matching records only, and LEFT JOIN to preserve all records from the primary left table.
#SQL#Joins#INNER JOIN#LEFT JOIN#Beginner
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