Relational Division Without Remainder: Finding Parent Table Entries with Exact Same Values and Number of Child Table Entries

Introduction

The question in the provided Stack Overflow post is about finding parent table entries that have the same values and the same number of child table entries. The goal is to retrieve parents with matching criteria from a larger set. This problem falls under the category of relational division without remainder, where we aim to eliminate non-relevant rows while maintaining the desired relationships.

Problem Statement

We are given two tables: Parent and Child, with a 1-to-many relationship between them. The table names are shown in the provided question, along with sample data. We need to write a SQL query that takes a specific parent ID as input and returns all matching parents from the main set.

Understanding Relational Division Without Remainder

Relational division without remainder is a technique used for eliminating rows in tables based on their relationships with other tables. In this case, we’re dealing with two tables: Parent and Child. We want to find all Parent entries that have matching values (specifically, Value_A) when compared to the number of child rows (Count_BIG(*)). The query must also ensure that all child rows match as well.

Solution Overview

There are several approaches to solving this problem. In this response, we’ll focus on a classic approach using SQL with joins and aggregations.

SQL Query

DECLARE @yourId int = 1;
DECLARE @count bigint = (SELECT COUNT(*) FROM Child WHERE ParentId = @yourId);

SELECT *
FROM Parent p
WHERE p.Id <> @yourId
  AND EXISTS (SELECT 1
    FROM Child other
    LEFT JOIN Child cThis
       ON cThis.Value_a = other.Value_a
      AND cThis.Value_b = other.Value_b
      AND cThis.ParentId = @yourId
    WHERE other.ParentId = p.Id
    HAVING COUNT_BIG(*) = @count                     -- same number of matches
       AND COUNT_BIG(*) = COUNT_BIG(cThis.ParentId)  -- all rows match
);

Explanation

1. We first declare two variables: @yourId for the input parent ID and @count to store the total count of child rows associated with the given parent ID.
2. In the main query, we select all rows from the Parent table where the ID is different from the specified parent ID (p.Id <> @yourId). We also use an existence check to ensure that for each matching parent row in the result set, there exist child rows with the same values and matching counts.
3. The existence condition uses a LEFT JOIN between the original Child table (other) and the Parent table (cThis) on both columns of interest (Value_a and Value_b). We match any parent ID present in the main set using this joined data structure.

Optimization Techniques

For performance optimization purposes, we can consider implementing additional techniques:

1. Indexing: Create indexes on columns used for joins (e.g., Value_a, Value_b) to improve join speeds.
2. Subqueries or Views: Use subqueries or views to store and recompile these common queries if needed multiple times.

Relational Division With Remainder

In cases where you want all rows from both tables that satisfy the conditions, not just those from the main set, simply replace LEFT JOIN with INNER JOIN, but update your HAVING clause like this:

    HAVING COUNT_BIG(*) >= @count   -- at least that number of matches

Conclusion

The question in the provided Stack Overflow post is an example of relational division without remainder. The query has been explained, and optimization techniques have been suggested to improve performance.

By applying these principles, you should be able to find all Parent entries matching the specified criteria in your own SQL queries.

Last modified on 2024-07-08