Understanding PostgreSQL Joins and Duplicate Rows

PostgreSQL is a powerful object-relational database management system that supports various types of joins, including INNER JOINs, LEFT JOINs, RIGHT JOINs, and FULL OUTER JOINs. In this article, we will explore how to eliminate duplicate rows in PostgreSQL join operations.

The Problem: Duplicate Rows in Joins

In the provided Stack Overflow question, a user is attempting to join three tables using LEFT JOINs to retrieve data from the MEAL table along with related information from the INGREDIENT and FLAVOR tables. However, the query results contain duplicate rows due to the duplication of flavors.

The Query: A Close Look

The provided SQL query appears to be syntactically correct but may not produce the desired result without further modifications:

SELECT 
   meal.id,
   meal.name,
   JSON_AGG(i) as ing,
   JSON_AGG(f) as flav,
FROM meal LEFT JOIN 
   (SELECT ingredient.id, ingredient.name 
    FROM ingredient) i 
      ON (i.id = ANY(meal.ingredients)) 
LEFT JOIN 
   (SELECT flavor.id, flavor.name 
    FROM flavor) f 
      ON (f.id = ANY(meal.flavors))
GROUP BY 
   meal.id,
   meal.name

This query uses LEFT JOINs to combine data from the MEAL table with related information from the INGREDIENT and FLAVOR tables. However, the use of JSON_AGG function with duplicates may lead to unwanted results.

Why Does This Happen?

When using JSON_AGG to aggregate duplicate values, PostgreSQL will repeat each value once, resulting in duplicate rows in the final result set.

Solution: Eliminating Duplicate Rows Using Distinct

To eliminate duplicate rows in the join operation, we can use the DISTINCT keyword or the GROUPING SETS clause. In this case, we’ll utilize GROUPING SETS.

Using GROUPING SETS

The GROUPING SETS clause allows us to group rows by one or more groups using parentheses, effectively excluding duplicate values.

SELECT 
   meal.id,
   meal.name,
   JSON_AGG(DISTINCT i) as ing,
   JSON_AGG(DISTINCT f) as flav,
FROM meal LEFT JOIN 
   (SELECT ingredient.id, ingredient.name 
    FROM ingredient) i 
      ON (i.id = ANY(meal.ingredients)) 
LEFT JOIN 
   (SELECT flavor.id, flavor.name 
    FROM flavor) f 
      ON (f.id = ANY(meal.flavors))
GROUP BY 
   GROUPING SETS (
       (meal.id, meal.name),
       ()
   )

In this modified query, we’re grouping by the meal.id and meal.name columns separately from an empty group. This effectively removes duplicate rows.

Using DISTINCT

Another approach is to use the DISTINCT keyword within the JSON_AGG function:

SELECT 
   meal.id,
   meal.name,
   JSON_AGG(DISTINCT ingredient.name) as ing,
   JSON_AGG(DISTINCT flavor.name) as flav,
FROM meal LEFT JOIN 
   (SELECT ingredient.id, ingredient.name 
    FROM ingredient) i 
      ON (i.id = ANY(meal.ingredients)) 
LEFT JOIN 
   (SELECT flavor.id, flavor.name 
    FROM flavor) f 
      ON (f.id = ANY(meal.flavors))
GROUP BY 
   meal.id,
   meal.name

This query uses DISTINCT to remove duplicate values within the JSON_AGG function.

Conclusion

In this article, we explored how to eliminate duplicate rows in PostgreSQL join operations using GROUPING SETS and the DISTINCT keyword. By applying these techniques, you can efficiently retrieve data from related tables without unwanted duplicates.

Last modified on 2023-10-28