Oracle

In Oracle SQL, a LEFT JOIN (or LEFT OUTER JOIN) is used to combine rows from two or more tables based on a related column between them. It returns all records from the left table and the matched records from the right table. If there is no match, the result is NULL on the side of the right table. This is particularly useful when you want to retrieve all entries from one table, along with any corresponding entries from another table.

Here's how you can implement a LEFT JOIN in Oracle:

SELECT a.author_name, b.book_title
FROM authors a
LEFT JOIN books b ON a.author_id = b.author_id;

Key Points about LEFT JOIN

1. Definition

   • Combines rows from two tables based on a related column.
   • Returns all records from the left table and matched records from the right table.

2. Syntax

   • Basic syntax structure:

     SELECT columns
     FROM left_table
     LEFT JOIN right_table ON left_table.column = right_table.column;
     

3. Use Cases

   • Useful for reporting and retrieving comprehensive data from primary tables while still showing related records.
   • Ideal for scenarios where you need to display all records from one table regardless of their relationship with another.

4. Example

   • To get a list of all authors and their books (including authors without books):

     SELECT a.author_name, b.book_title
     FROM authors a
     LEFT JOIN books b ON a.author_id = b.author_id;
     

5. Best Practices

   • Ensure that your join conditions are clear to prevent unexpected results.
   • Use aliases for table names to improve readability.
   • Always check for NULL values in the results from the right table to handle unmatched rows appropriately.

Additional Considerations

• Performance

  • Be mindful of the potential performance implications when joining large tables; use indexes where appropriate.

• Data Integrity

  • Ensure referential integrity to maintain the relationships between tables.

By understanding and utilizing LEFT JOIN, you can effectively manage data retrieval across multiple tables in Oracle SQL, ensuring comprehensive and insightful data analysis.

To gain complete access, login with gmail or outlook, no need of signup. click here

SELECT prd_product.product_id,
       prd_product.product_code,
       prd_product.product_name,
       act_order_detail.order_id
FROM prd_product
LEFT JOIN act_order_detail ON act_order_detail.product_id = prd_product.product_id
ORDER BY prd_product.product_id, act_order_detail.order_id;

SELECT a.order_number,
       a.order_date,
       b.payment_id
FROM act_order a
LEFT JOIN act_payment b ON b.order_id = a.order_id
ORDER BY a.order_number;

-- To filter orders that have not received ANY payments yet
SELECT
    a.order_number,
    a.order_date
FROM act_order a
LEFT JOIN act_payment b ON b.order_id = a.order_id
WHERE b.order_id IS NULL;

SELECT org_client.client_id,
       org_client.first_name AS client_name,
       SUM(act_order.sub_total) AS lifetime_order_amount
FROM org_client
LEFT JOIN act_order ON act_order.client_id = org_client.client_id
GROUP BY org_client.client_id, org_client.first_name
ORDER BY org_client.client_id;

EXAMPLE 1 - SQL LEFT JOIN

Here is a clear example of a LEFT JOIN. Retrieve information of all products along with their respective order details, including products that have not been associated with any orders.

EXAMPLE 1 - Tansy Academy Data Model

In this task, you will create a query involving two tables, named Products and Order details, marked are the columns necessary for the query.

EXAMPLE 1 - LEFT JOIN query

To achieve this, you need to execute a LEFT JOIN between the product table and the order detail table, utilizing the primary key and foreign key column, wherein the product ID column serves as the joining column. In this scenario, the product table functions as the left table, and the order detail table is considered the right table. The left table, which contains the essential primary business information, focuses on the product list as the primary requirement. At a secondary level, we seek order IDs, so we treat the order details as the secondary table, positioned on the right side of the join

SELECT prd_product.product_id, prd_product.product_code, prd_product.product_name
    act_order_detail.order_id
FROM prd_product
LEFT JOIN act_order_detail ON act_order_detail.product_id = prd_product.product_id
ORDER BY prd_product.product_id, act_order_detail.order_id;

EXAMPLE 1 - Query Data Mapping

In this visual representation, a yellow background denotes a correspondence between the primary key and foreign key. Meanwhile, a light yellow background with red font on the left side signifies products lacking a corresponding row in the order details table. These products will be included in the final result, but with NULL values for order IDs. It's important to recognize that a LEFT JOIN incorporates all rows from the left table, which, in this case, is the product table.

EXAMPLE 1 - Final OUTput

In this view, it's evident that certain product records from the left table have multiple corresponding rows on the right side. Consequently, the product information will be associated with each order entry from the right side.