Workarounds for Combining LEFT JOINs and UNION in MariaDB

Scenarios and Workarounds:

1. Union results from separate LEFT JOINs:

   • If you want to combine data from two independent LEFT JOIN queries, you can write them as separate UNION statements.

   Example: Find products with quantity > 0 from two warehouses (tables warehouse1 and warehouse2).

   SELECT product_id, quantity
   FROM warehouse1
   LEFT JOIN products p ON warehouse1.product_id = p.id
   WHERE quantity > 0
   
   UNION
   
   SELECT product_id, quantity
   FROM warehouse2
   LEFT JOIN products p ON warehouse2.product_id = p.id
   WHERE quantity > 0
   

2. Perform LEFT JOIN after UNION:

   • If you want to join data after combining results from multiple SELECT statements (not necessarily using LEFT JOIN), you can use a subquery with UNION and then perform the LEFT JOIN on the combined data.

   Example (Less common): Find all customers (even without purchases) and their recent purchase (if any).

   (SELECT customer_id, NULL AS product_name, NULL AS purchase_date
   FROM customers
   
   UNION ALL
   
   SELECT c.customer_id, p.name AS product_name, MAX(o.purchase_date) AS purchase_date
   FROM customers c
   LEFT JOIN orders o ON c.customer_id = o.customer_id
   LEFT JOIN products p ON o.product_id = p.id
   GROUP BY c.customer_id) AS all_data
   
   LEFT JOIN products p ON all_data.product_name = p.name;  -- Join after UNION
   

Key takeaway:

• Use a subquery with UNION for complex scenarios where you join data after combining results from multiple queries.
• Use separate UNION statements for combining results from independent LEFT JOIN queries.

Example Codes for UNION with LEFT JOIN Workarounds in MariaDB

Scenario 1: Combining results from separate LEFT JOINs

-- Find products with quantity > 0 from warehouse1
SELECT product_id, quantity
FROM warehouse1
LEFT JOIN products p ON warehouse1.product_id = p.id
WHERE quantity > 0

UNION

-- Find products with quantity > 0 from warehouse2
SELECT product_id, quantity
FROM warehouse2
LEFT JOIN products p ON warehouse2.product_id = p.id
WHERE quantity > 0;

Scenario 2: Performing LEFT JOIN after UNION

-- Combine customer data (all) and recent purchases (if any)
(SELECT customer_id, NULL AS product_name, NULL AS purchase_date
FROM customers

UNION ALL

SELECT c.customer_id, p.name AS product_name, MAX(o.purchase_date) AS purchase_date
FROM customers c
LEFT JOIN orders o ON c.customer_id = o.customer_id
LEFT JOIN products p ON o.product_id = p.id
GROUP BY c.customer_id) AS all_data

-- Join all_data with products table to get product name (if purchased)
LEFT JOIN products p ON all_data.product_name = p.name;

1. EXISTS Clause with LEFT JOIN:

This method uses the EXISTS clause to check if a record exists in another table based on a condition. It can be used to selectively include rows from a LEFT JOIN based on another table.

Example: Find all products with at least one order (replacing LEFT JOIN with EXISTS and LEFT JOIN for comparison).

Original (LEFT JOIN):

SELECT p.name AS product_name
FROM products p
LEFT JOIN orders o ON p.id = o.product_id;

Alternate (EXISTS):

SELECT name AS product_name
FROM products p
WHERE EXISTS (
  SELECT 1
  FROM orders o
  WHERE o.product_id = p.id
);

1. CASE Statement with LEFT JOIN:

This method uses a CASE statement to conditionally include data based on the existence of a match in a LEFT JOIN.

Example: Find product details and a message indicating if the product has any orders (replacing LEFT JOIN with CASE).

SELECT p.name AS product_name, o.id AS order_id
FROM products p
LEFT JOIN orders o ON p.id = o.product_id;

SELECT p.name AS product_name,
  CASE WHEN o.id IS NULL THEN 'No Orders' ELSE o.id END AS order_information
FROM products p
LEFT JOIN orders o ON p.id = o.product_id;

1. CTE (Common Table Expression):

This method involves creating a temporary named result set (CTE) using a LEFT JOIN. You can then use this CTE in your main query for further processing.

Example: Find total quantity of each product across all warehouses (assuming a table warehouse_products with product_id, warehouse_id, and quantity).

WITH warehouse_data AS (
  SELECT product_id, SUM(quantity) AS total_quantity
  FROM warehouse_products
  GROUP BY product_id
)

SELECT p.name AS product_name, wd.total_quantity
FROM products p
LEFT JOIN warehouse_data wd ON p.id = wd.product_id;