How to Use JOINs in MariaDB

How do I use JOINs in MariaDB to combine ecommerce tables?

JOINs combine rows from two or more tables based on related columns, enabling richer result sets.

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Description

What problem do JOINs solve?

JOINs let you read related data spread across multiple tables in one query, eliminating costly application-side stitching.

Which JOIN types exist in MariaDB?

MariaDB supports INNER, LEFT (OUTER), RIGHT (OUTER), CROSS, and self-JOIN.Use each based on whether unmatched rows should appear.

When should I use INNER JOIN?

Use INNER JOIN to return only rows with matching keys in both tables—for example, customers who have placed orders.

How does LEFT JOIN differ from INNER JOIN?

LEFT JOIN keeps every row from the left table even if no match exists on the right; unmatched columns become NULL.

Example: list all customers, even those without orders

SELECT c.id, c.name, o.id AS order_id FROM Customers c LEFT JOIN Orders o ON o.customer_id = c.id;

What is a RIGHT JOIN used for?

RIGHT JOIN is the mirror of LEFT JOIN but less common—it preserves every row from the right table.

How do I join more than two tables?

Chain JOIN clauses.MariaDB processes them left to right, so use parentheses only when mixing INNER and OUTER JOIN priorities.

Example: customers, orders, and order totals

SELECT c.name, o.id, SUM(oi.quantity * p.price) AS total FROM Customers c INNER JOIN Orders o ON o.customer_id = c.id INNER JOIN OrderItems oi ON oi.order_id = o.id INNER JOIN Products p ON p.id = oi.product_id GROUP BY c.name, o.id;

Why avoid old comma joins?

Comma joins mix join and filter logic in WHERE, hiding outer join capability and increasing risk of accidental cross joins.

Best practices for JOINs?

Index foreign keys, qualify column names, return only needed columns, and profile queries with EXPLAIN.

Why How to Use JOINs in MariaDB is important

How to Use JOINs in MariaDB Example Usage


-- Find products purchased by a given customer in the last 30 days
SELECT p.id, p.name, p.price, oi.quantity
FROM   Customers  AS c
INNER  JOIN Orders      AS o  ON o.customer_id = c.id
INNER  JOIN OrderItems  AS oi ON oi.order_id   = o.id
INNER  JOIN Products    AS p  ON p.id          = oi.product_id
WHERE  c.email = 'alice@example.com'
  AND  o.order_date >= CURRENT_DATE - INTERVAL 30 DAY;

How to Use JOINs in MariaDB Syntax


-- Basic syntax
SELECT select_list
FROM   table1 [AS alias1]
{ INNER | LEFT [OUTER] | RIGHT [OUTER] | CROSS } JOIN table2 [AS alias2]
ON     join_condition
[... additional JOIN clauses ...]

-- Ecommerce examples
-- 1. INNER JOIN Customers & Orders
SELECT c.name, o.order_date, o.total_amount
FROM   Customers c
INNER JOIN Orders o ON o.customer_id = c.id;

-- 2. LEFT JOIN Orders & OrderItems to find empty carts
SELECT o.id, oi.id AS item_id
FROM   Orders o
LEFT JOIN OrderItems oi ON oi.order_id = o.id
WHERE  oi.id IS NULL;

Common Mistakes

Using SELECT * with multiple JOINs inflates I/O and network cost. Explicitly pick required columns to boost speed and readability.
Forgetting an ON clause in CROSS joins unintentionally creates a Cartesian product, multiplying rows and crashing performance. Always specify join conditions unless a true cross is needed.