Скачать или смотреть How to Use INSERT ... SELECT with the WITH Clause in Postgres

Discover how to efficiently use the `INSERT ... SELECT` statement with a `WITH` clause in Postgres, tackling common pitfalls and errors.
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Mastering Postgres INSERT ... SELECT with the WITH Clause

When working with PostgreSQL, one fertile area that can sometimes be confusing is the use of the INSERT ... SELECT statement in conjunction with a WITH clause. In this guide, we'll explore a common problem involving inserting data into tables while also utilizing results from previous insert operations. If you’ve encountered an error when attempting to do this, don’t worry—you're not alone! Let's dive into the problem and break down the solution step-by-step.

The Problem

A user was trying to create multiple entries in a dog table using an INSERT ... SELECT statement, and also wanted to include the generated primary key from those inserts into a secondary dog_toys table. They encountered an error that read:

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

This issue surfaced because the user was trying to select from the results of a WITH clause that returned multiple rows, which PostgreSQL doesn’t allow as a singular expression within a subquery. The main challenge, therefore, is to find a way to handle multiple returns from the WITH clause effectively.

The Solution

Understanding the Error

The specific error stems from using (SELECT dog_uuid FROM dogEntries) in the SELECT portion of the second INSERT statement. Since dogEntries may return multiple rows, PostgreSQL throws an error, as it's expecting a single value.

A Revised Approach

To resolve this issue, we can modify our SQL statement by employing joins instead of subqueries. Here’s how to structure it correctly:

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

Breaking It Down

Using the WITH Clause: The WITH clause creates a CTE (Common Table Expression) named dogentries, which inserts new rows into the dog table and returns their IDs.

Inserting into dog_toys: The second INSERT INTO dog_toys statement selects from dogentries (which holds the new dog IDs) and also joins with dog and dog_toys tables to retrieve the necessary data for the insertion.

Joining Properly: By using proper joins, we can seamlessly incorporate multiple values returned from dogentries into the new dog_toys records.

Things to Note

Ensure your table definitions allow for the intended relationships between dog, dog_toys, and any other tables referenced.

Adjust your queries according to your actual database schema and consider reviewing indexes on your tables for improved performance.

Conclusion

Using a WITH clause in PostgreSQL can simplify complex queries, but it comes with its caveats. Understanding how to handle multiple rows from such expressions will help prevent common errors like the one we discussed. The key takeaway is to leverage joins instead of subqueries when you're expecting multiple results.

Happy querying! If you have any further questions or run into challenges, feel free to reach out or leave a comment below.