Resolving the Multiple JOINs Issue in TSQL: A Case Study with Northwind Database

Discover how to handle `Multiple JOINs` effectively in TSQL when working with the Northwind Database. Learn to produce a full list of countries with accurate suppliers and customers count.
---
This video is based on the question https://stackoverflow.com/q/73687719/ asked by the user 'Michael Shuv' ( https://stackoverflow.com/u/12383407/ ) and on the answer https://stackoverflow.com/a/73689883/ provided by the user 'sgmoore' ( https://stackoverflow.com/u/125759/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Multiple JOINs issue in TSQL

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Resolving the Multiple JOINs Issue in TSQL: A Case Study with Northwind Database

When working with databases, especially in SQL, it’s common to encounter issues related to data retrieval involving JOIN operations. One such issue arises when trying to gather information from multiple tables that may have a common data field. In this guide, we’ll explore a specific problem related to multiple JOINs in TSQL, using the Northwind Database as our reference point.

The Problem Statement

In the Northwind Database, both Customers and Suppliers tables include a Country column. However, there is no separate Country table in the database, meaning that the Country field is not a foreign key.

The Task

The goal is to produce a comprehensive list of countries derived from both the Customers and Suppliers tables, accompanied by the total number of customers and suppliers for each country. Here’s the key requirement:

If a country has no customers or suppliers, it should display 0 instead of NULL.

The Specifics

For example, Argentina has three customers but no suppliers. When running the SQL query, the expected output should reflect:

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

However, what we encountered instead was:

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

Here’s where the confusion and issue lies.

The Solution

To solve this problem, let's break down the solution into manageable sections.

Step 1: Using Common Table Expressions (CTEs)

The solution employs Common Table Expressions (CTEs) to prepare the data sets:

CountryList: Unites the countries from both Customers and Suppliers.

SupplierCountry: Counts suppliers by country.

CustomerCountry: Counts customers by country.

SQL Code Breakdown

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

Here’s how each part works:

CountryList collects all unique country names from both tables, ensuring we don't miss any countries.

SupplierCountry counts the number of suppliers grouped by their country.

CustomerCountry performs a similar count for customers.

Step 2: JOIN Operations

The issue arises during the JOIN operation. Initially, the incorrect join was:

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

The Correction

To resolve the issue, it is important to change this line to:

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

Explanation

By using cl.Country, we ensure that every country listed in the CountryList will fetch the correct customer count, even if there are no suppliers.

This means that if there are no suppliers for a country, sc.Country will be NULL, effectively linking back to the CountryList.

Final SQL Query

Here’s the corrected final query:

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

Expected Outputs

With these adjustments, now when running the query for Argentina, the output reflects the accurate counts:

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

Conclusion

Handling multiple JOINs in SQL can be tricky, but understanding the underlying relationships and formulating the correct joins can significantly improve your data retrieval efforts. By ensuring that the join conditions are based on the primary table (in this case, CountryList), you can produce the desired results, retaining data integrity even when faced with NULL values.

Arming yourself with these techniques can streamline your SQL processes and ensure accurate reporting from relational databases.