How to Store a Query Result in a Variable for SQL Insert Operations

Learn how to use PostgreSQL to effectively manage inserts with dynamic primary keys using returning results without hardcoding values.
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Introduction

When working with relational databases like PostgreSQL, developers often face the challenge of inserting data sequentially from parent tables to child tables. A common requirement is to store the primary key of a newly inserted row in a variable, so it can be reused when inserting related data into another table. In this guide, we will explore if it is possible to store a query in a variable and utilize that variable in an INSERT query in PostgreSQL. We will break down the best solutions to achieve this.

The Problem

You may have encountered a scenario where you have already inserted data into a parent table, and you now want to save the primary key of that row in order to use it as a foreign key in a related child table:

You have inserted data into the countries table and need the id of that entry to insert into the cities table.

The query you've attempted looks something like this:

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

However, this raises questions: How can you store the result of a query into a variable and use it in another query? SQL does not support this syntax directly, so let's discuss the solution.

The Solution

Using INSERT ... RETURNING

In PostgreSQL, a straightforward way to achieve your goal is to leverage the INSERT ... RETURNING statement. This statement allows you to insert a row into a table and retrieve the generated primary key immediately, which you can then use for subsequent operations.

Here’s How It Works

Insert data into the parent table:

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

This command will insert a country with the description 'asdf' and return the generated id immediately.

Capture the returned value on the client side (e.g., in your application code). Assume the returned id is 4711.

Use the captured id for the child table insertion:

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

In this case, 4711 is the primary key value obtained from the first statement.

Alternative Approach: Using Common Table Expressions

If you prefer to run your insertions in a single SQL query, you can use a Common Table Expression (CTE):

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

This approach has the advantage of encapsulating the logic in one go, making it cleaner and easier to maintain.

Conclusion

Storing a query result in a variable to facilitate data insertion in SQL isn't straightforward due to SQL's design. However, by utilizing the INSERT ... RETURNING statement or leveraging Common Table Expressions, you can achieve the desired outcome effectively without hardcoding primary key values.

Both methods discussed provide flexible ways to handle relationships between tables while ensuring data integrity. Feel free to choose the option that best suits your application requirements!

By using these strategies, you can streamline your data operations in PostgreSQL and overcome the challenges of working with parent-child table relationships efficiently.