Finding Distinct Pairs in SQL: A Closer Look at Non-Equi Joins

In this article, we will delve into the world of non-equi joins and explore how to find distinct pairs in a SQL query. We will examine the provided example, discuss the common pitfalls, and provide practical advice on how to improve performance and accuracy.

Understanding Non-Equi Joins

A non-equi join is a type of join that does not match rows based solely on equality conditions between columns. Instead, it uses non-equality operators like BETWEEN, IN, or LIKE to determine whether two rows should be joined. This allows us to perform joins with complex predicates and filter out unwanted data.

The provided example uses a non-equi join with the BETWEEN operator to find pairs of countries with populations within 100 units of each other.

SELECT DISTINCT 
       p1.country, 
       p2.country, 
       p1.population, 
       p2.population 
FROM pops p1 
INNER JOIN pops p2 
     ON p1.population BETWEEN p2.population - 100 AND p2.population + 100 
     AND p1.country < p2.country  
WHERE p2.country > p1.country

Analysis of the Provided Example

Let’s break down the provided example and analyze its strengths and weaknesses.

The query uses a non-equi join with BETWEEN to find pairs of countries with populations within 100 units of each other.
It also includes an additional condition using the < operator to ensure that the country in p1 comes before the country in p2.
Finally, it filters out unwanted data by only including pairs where the country in p2 appears after the country in p1.

However, there’s a catch!

Pitfall: Redundant Where Clause

The query has an unnecessary WHERE clause that filters out additional rows. Since we’ve already ensured that p1.country < p2.country, this filter is redundant and can be removed.

Improved Query

SELECT DISTINCT 
       p1.country, 
       p2.country, 
       p1.population, 
       p2.population 
FROM pops p1 
INNER JOIN pops p2 
     ON p1.population BETWEEN p2.population - 100 AND p2.population + 100 
     AND p1.country < p2.country

Simplifying the Query with Lexicographical Order

As the answer suggests, we can improve the query by using the LEAXICAL ORDER approach.

We add the LEAXICAL ORDER keyword to ensure that the comparison is done lexicographically (i.e., alphabetically).
By doing so, we guarantee that the country in p1 comes before the country in p2, which eliminates duplicate pairs.

Improved Query with Lexicographical Order

SELECT DISTINCT 
       p1.country, 
       p2.country, 
       p1.population, 
       p2.population 
FROM pops p1 
INNER JOIN pops p2 
     ON p1.population BETWEEN p2.population - 100 AND p2.population + 100 
     AND LEAXICAL ORDER(p1.country, p2.country) = 'p2.country'

Best Practices for Finding Distinct Pairs

Based on our analysis and example queries, here are some best practices to keep in mind when finding distinct pairs in SQL:

Avoid Redundant Where Clauses: Ensure that any additional filtering conditions are included within the join predicate or as a separate condition.
Use Lexicographical Order: When necessary, use the LEAXICAL ORDER keyword to guarantee lexicographical ordering and eliminate duplicate pairs.
Optimize Join Predicates: Carefully craft your join predicates to minimize the number of rows being joined and filtered.
Test and Validate: Verify that your query produces the expected results using sample data.

Conclusion

Finding distinct pairs in SQL requires a deep understanding of non-equi joins, lexicographical ordering, and optimization techniques. By following best practices and avoiding common pitfalls, you can create efficient and accurate queries to uncover valuable insights from your data.

In this article, we explored the provided example and demonstrated how to improve performance and accuracy using improved queries and best practices. We hope that this analysis has provided a deeper understanding of non-equi joins and their applications in SQL.

Last modified on 2023-08-04