Database.Beginner.What is the difference between INNER JOIN and LEFT JOIN?

INNER JOIN:

• Returns only the rows that have matching values in both tables.
• If there’s no match between the two tables, that row is excluded from the result.

LEFT JOIN (or LEFT OUTER JOIN):

• Returns all rows from the left table.
• If there’s a match in the right table, it returns the matching row.
• If there’s no match, it still returns the row from the left table, but with NULLs for columns from the right table.

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Example

Given two tables:

Customers

customer_id	name
1	Alice
2	Bob
3	Charlie

Orders

order_id	customer_id	product
101	1	Laptop
102	2	Smartphone

INNER JOIN example:

SELECT Customers.name, Orders.product
FROM Customers
INNER JOIN Orders ON Customers.customer_id = Orders.customer_id;

Result:

name	product
Alice	Laptop
Bob	Smartphone

• Only Alice and Bob appear — because they have matching orders.
• Charlie doesn’t have an order, so he’s not shown.

LEFT JOIN example:

SELECT Customers.name, Orders.product
FROM Customers
LEFT JOIN Orders ON Customers.customer_id = Orders.customer_id;

Result:

name	product
Alice	Laptop
Bob	Smartphone
Charlie	NULL

• Alice and Bob have matching orders — so we see their orders.
• Charlie has no matching order — but he still shows up, and product is NULL.

In Short:

JOIN Type	Rows Returned
INNER JOIN	Only where there is a match in both tables.
LEFT JOIN	All from left table, matched or not (fill NULLs if no match).

🎯 Analogy:

• INNER JOIN is like an intersection — show only what both have in common.
• LEFT JOIN is like a left sweep — show everything from the left, and match from the right if you can, otherwise leave NULL.