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Learn how to effectively incorporate custom values into your SQL queries by adding an extra column, even when working with separate databases in PostgreSQL.
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How to Add Custom Values as an Additional Column in SQL Queries

When working with SQL, there may be times when you need to enhance the output of your queries by adding supplementary information. One common scenario involves adding a list of custom values as a new column to an existing SQL query. This post will walk you through how to achieve this in PostgreSQL, specifically addressing a situation where data is located in different databases, making it impossible to join tables directly.

The Problem

Imagine you have executed the following SQL query:

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

This query returns a table that lists the number of distinct users who submitted data each month. Here’s a simplified view of the output:

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

Now, you would like to add a third column with custom values, resulting in:

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

The Solution

Since you need to include custom values and cannot join directly due to the separation of databases, you can utilize a combination of PostgreSQL's JOIN clause with VALUES.

Here's how to structure your query:

1. Utilizing VALUES in a Subquery

You will first create a subquery that consists of your custom values aligned to the months. The syntax goes as follows:

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

2. Explanation of the Query

Subquery with VALUES: Here, we create a temporary table using the subquery, which holds your custom values alongside the corresponding month.

LEFT JOIN: This allows us to join the main query with the values defined in the subquery based on the condition that the truncated month matches.

Grouping and Ordering: We group by the truncated date and the custom value to ensure accurate results, while ordering by date ensures your output is easy to read chronologically.

3. Benefits of This Approach

No New Tables Required: You don't need to create any new tables in the database, maintaining a clean database environment.

Flexibility: This method can be easily adapted for various cases where custom values change, just by modifying the VALUES clause.

Conclusion

Adding a new column to your SQL query outputs does not have to be a complicated process, even when the data is spread over different databases. By using a LEFT JOIN with VALUES, you can seamlessly incorporate custom values into your results.

Give this approach a try, and you'll find it a powerful tool for enhancing your SQL queries.