How to Fix the FULL JOIN Error in Amazon Redshift

Описание: Learn how to resolve the error "FULL JOIN is only supported with merge-joinable join conditions" in Amazon Redshift when combining tables with specific join conditions.
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This video is based on the question https://stackoverflow.com/q/62712039/ asked by the user 'Cribber' ( https://stackoverflow.com/u/3909896/ ) and on the answer https://stackoverflow.com/a/62714683/ provided by the user 'Gordon Linoff' ( https://stackoverflow.com/u/1144035/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the FULL JOIN Error in Amazon Redshift

If you’re working with Amazon Redshift and attempting to perform a FULL OUTER JOIN, you might encounter the error:

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

This error can be quite frustrating, especially when you need to combine data from multiple tables in your database.

In this guide, we’ll delve into what causes this error and provide a step-by-step solution to overcome it.

The Scenario

Imagine you have two tables in your Amazon Redshift database. Here’s a brief overview of their structure:

Table 1 (e.g., table1):

id

colx (date)

Table 2 (e.g., table2):

id

col2 (date range beginning)

col3 (date range end)

something

You’re trying to retrieve all records from both tables, even if there are no matches on the join condition, which is why you want to perform a FULL OUTER JOIN. However, you encounter the error when executing the following SQL statement:

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

The Cause of the Error

The reason for the error lies in the joining condition you are using. Amazon Redshift mandates that a FULL OUTER JOIN must involve merge-joinable conditions, which often isn't the case when you are including complex conditions—such as ranges or using functions on columns (like ltrim()).

Solution: Using UNION ALL

An effective workaround to the FULL JOIN error is to utilize UNION ALL. Here’s how you can do this:

Step 1: Perform a LEFT JOIN

The first part of the solution is to create a LEFT JOIN between the two tables:

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Step 2: Use Another LEFT JOIN

Next, perform another LEFT JOIN in the opposite direction and filter for conditions where there are no matches:

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

Step 3: Combine with UNION ALL

Finally, combine both queries using the UNION ALL operator:

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

Conclusion

By using UNION ALL, you can effectively achieve the result of a FULL OUTER JOIN without running into the merge-joinable condition error imposed by Amazon Redshift. This approach will allow you to retrieve all records from both tables while accounting for unmatched records as intended.

Happy querying, and may your database handle all your needs seamlessly!