Скачать или смотреть How to Return a Value from a PostgreSQL Delete that Impacts 0 Rows?

Discover how to effectively return a value from a PostgreSQL delete operation, even when it affects no rows, by using a clever SQL technique.
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This video is based on the question https://stackoverflow.com/q/71026632/ asked by the user 'Oliver Rice' ( https://stackoverflow.com/u/6312218/ ) and on the answer https://stackoverflow.com/a/71027316/ provided by the user 'Ramin Faracov' ( https://stackoverflow.com/u/17296084/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem: Returning Values in PostgreSQL Delete Operations

PostgreSQL is a powerful database that allows for intricate operations, including deleting records from a table. However, a common question developers face is, can you return a value from a delete statement when no rows are actually deleted?

Consider the following scenario:

You attempt to delete a record with an id of 999 from a table called dummy. However, if there are no rows with that id, the deletion operation affects 0 rows. In such cases, utilizing the RETURNING clause simply returns null, which is not ideal.

Let's explore how we can solve this problem by creatively using an SQL feature called UNION.



Solution: Using UNION to Handle No Deleted Rows

Though the RETURNING clause typically focuses on deleted records, we can use a logical approach to ensure a return value even when no records are deleted. Here’s how:

Step-by-step Breakdown

Understand the Delete Command:

When you execute a delete statement with the RETURNING clause, PostgreSQL will return the records that were actually deleted. If no records are deleted, it returns null.

Introducing the UNION Command:

To circumvent this limitation, we can use the UNION command. This allows us to add an additional record to the results of our deletion.

Crafting the Query:

We can create a Common Table Expression (CTE) to execute the delete command. Then, we use a UNION ALL to ensure that we always get at least one record back, regardless of whether any rows were deleted.

Example Query

Here’s an example of how you can implement this:

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

Explanation of the Example

CTE (del_table): This section performs the delete operation and attempts to return the id of the deleted record(s).

UNION ALL: This command combines results. If the delete operation didn't affect any rows, it will still return 1 from the second select statement, thus providing a default value instead of null.

Sample Output

When using the above query:

If the record was deleted, you'd receive the id of the deleted row and the value 1.

If no matching record existed, you'd receive just 1 as output.



Conclusion

By utilizing the UNION command with a CTE, you can effectively return a predefined value when a delete operation does not affect any rows. This approach enhances your SQL queries and helps maintain reliable returns, allowing for smoother application logic and error handling.

Next time you encounter the issue of a PostgreSQL delete operation returning null, remember this innovative solution!