Скачать или смотреть Resolving the more than one row returned by a subquery issue in PostgreSQL

A comprehensive guide to finding the youngest generation in a family tree using PostgreSQL, including solutions for handling subqueries that return multiple rows.
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Resolving the more than one row returned by a subquery issue in PostgreSQL

Fetching meaningful data from a relational database can sometimes present unique challenges. If you're working with PostgreSQL and have encountered the error "more than one row returned by a subquery used as an expression," you're not alone. This guide addresses this common problem by illustrating a solution to identify the youngest generation in a family tree data structure efficiently.

Understanding the Problem

In many hierarchical data structures, such as family trees, you may need to identify specific generations based on parent-child relationships. For instance, consider the following database structure:

IDNAMEPARENT_ID1A02B13C14D25E36F37G6You need to retrieve the youngest members of the family tree based on the number of generations. If there’s more than one youngest member, your output should show all of them along with their generation count. If there’s only one, ensure that it reflects correctly too.

Example Outputs

For multiple members:

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

For a single member:

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

Your initial query attempted to fulfill this requirement but resulted in a subquery returning multiple rows, which isn't permitted in certain contexts. Let's dissect how to resolve this problem.

The Solution: Recursive Common Table Expressions (CTEs)

The solution involves using a recursive Common Table Expression (CTE) to traverse the family tree while also counting generations. This approach allows you to dynamically capture generation levels while building the members' records.

Step-by-Step Implementation

Here’s a revised version of your query using a recursive CTE to address the issue:

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

Explanation of the Query

Recursive CTE Definition:

The CTE, named children, starts with selecting family members that are "leaf" nodes (nodes without children).

It initializes the generation column to 1.

It then recursively joins on itself to accumulate the children and increment the generation count.

Final Selection:

The outer query selects all records from the children CTE where the generation number is equal to the maximum generation derived from the CTE itself.

What This Achieves

By employing the recursive CTE approach, you effectively handle cases where there can be multiple records returned without experiencing the subquery limitation in PostgreSQL. This means you can collect all relevant members of the youngest generation efficiently.

Alternative Approach: Using Window Functions

Another interesting way to find the youngest generation without using a recursive CTE is to utilize PostgreSQL's window functions. With window functions, you can rank generations dynamically and select from that.

Here’s an example:

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

Recap

Using either the recursive CTE or window functions allows you to overcome the limitations of subqueries returning multiple rows. This approach ensures that you're efficiently fetching all the relevant data while adhering to PostgreSQL's constraints.

Conclusion

Navigating the complexities of SQL queries can be intimidating, but with the right techniques, such as recursive CTEs or window functions, you can solve even the trickiest issues like "more than one row returned by a subquery." The method shown in this post will help you effectively identify the youngest generation in a family tree structure, whether it includes one member or many.

Remember, understanding the structure of your data and the tools at your disposal is crucial for successful database