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Learn different techniques to `join multiple tables` in Oracle SQL, focusing on left joins across purchases, refunds, and finance batching tables.
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How to Effectively Join Multiple Tables in Oracle SQL

When working with databases, especially in SQL, it's common to encounter the need to join multiple tables to retrieve comprehensive information. In this blog, we will explore how to effectively join multiple tables in Oracle SQL, particularly focusing on a scenario involving purchases, refunds, and a finance batching table. We'll discuss various approaches to accomplish this, specifically focusing on utilizing left joins to get the desired output.

Understanding the Problem

You have three tables that are crucial for tracking orders and refunds:

Purchases Table (p): Contains details of the orders including order numbers and IDs.

Refunds Table (r): Holds information about refunds associated with orders.

Finance Batching Table (b): This table links to both purchases and refunds and contains the generic_id that can relate to different entities.

Table Structure

Here's a quick overview of the assumed structure of each table:

Purchases:

order_number (VARCHAR(10))

order_id (NUMBER)

order_date (DATE)

Refunds:

refund_id (NUMBER)

order_id (NUMBER, relates to purchases)

refund_date (DATE)

Send to Finance Batch:

generic_id (NUMBER, links to either order ID or refund ID)

type (TEXT)

status_reason (TEXT)

status_id (NUMBER)

The Objective

The aim is to fill in the order_number from the purchases table into the outputs generated from the finance batching table, which currently only shows a partial intersection of data.

Solutions for Joining Multiple Tables

Let's explore three different methods to join these tables in the desired way:

Option 1: Using Union

While a UNION operation combines results from multiple queries, it's essential to acknowledge that rows which don't meet the join criteria will not appear in the result set. Here’s how to implement it:

[[See Video to Reveal this Text or Code Snippet]]

Option 2: Join Purchases Multiple Times with COALESCE

Using the COALESCE function allows us to return the first non-null value from multiple joins. This approach can help us keep all rows, avoiding potential data loss from the UNION operation:

[[See Video to Reveal this Text or Code Snippet]]

Option 3: Use COALESCE in the Join Condition

Another approach is to utilize COALESCE in the join condition itself, which allows us to link records directly by favoring one ID over another based on availability:

[[See Video to Reveal this Text or Code Snippet]]

Final Thoughts

Joining multiple tables in Oracle SQL can be managed effectively through various methods depending on the structure of your data and the desired results. Each of the methods outlined—whether using UNION or COALESCE—serves to connect your purchases and refunds in a meaningful way. Choose the approach that best fits your specific scenario and ensure that you maintain the integrity of your data throughout the process.

By employing these techniques, you can easily pull together comprehensive financial insights, helping to streamline your operations and enhance efficiency. Happy querying!