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Discover how to filter orders in PostgreSQL based on specific conditions related to associated shipping logs. Learn step-by-step queries to achieve accurate results.
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How to Efficiently Select Orders with Specific Shipping Log Conditions in PostgreSQL

Managing databases effectively is crucial for any business, especially when dealing with orders and shipping logs. In this guide, we'll tackle a common problem that arises when you need to filter out orders based on the number of associated shipping logs and their specific stages. Let’s break down an example scenario and provide a comprehensive solution using SQL queries with PostgreSQL.

The Problem

Imagine you have two related tables: Orders and ShippingLogs. The structure of these tables is as follows:

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

Your requirement is to select the IDs from the Orders table where there is only one associated ShippingLog, and importantly, that log must have a stage of error. If there are two shipping logs or if the only shipping log has a stage of shipped, those orders should be excluded from the results.

Understanding the Initial Query

You might start by writing a query like the following:

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

While this query is a great start, it falls short because it checks only for the presence of an error log without confirming it's the only log associated with the order. The solution involves more detailed logic to surpass this limitation.

The Solution

Using Common Table Expressions (CTEs) and Analytic Functions

To achieve the desired results, we will employ a Common Table Expression (CTE) and utilize analytic functions such as COUNT() to classify and filter the data accurately. Here’s how the final query looks:

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

Explanation of the Query

Common Table Expression (CTE):

We create a temporary result set (cte) using the WITH clause.

This CTE selects necessary columns and computes two important counts:

num_logs: The total number of shipping logs per order.

non_error_cnt: The count of shipping logs that are not in the error stage.

Filtering Criteria:

In the final selection from the CTE, we specify a WHERE clause that filters down to orders with exactly one log and no logs that have a stage other than error.

Order by and Return:

Finally, we order the results by order ID and creation date for clarity and better organization.

Conclusion

By using a combination of CTEs and analytic functions, you can efficiently filter orders based on specific conditions related to their associated shipping logs in PostgreSQL. This method not only improves the accuracy of your query but also enhances its readability and maintainability.

We encourage you to experiment with this approach in your own database and see how it can streamline your order management processes. Happy querying!