ORA-00920: Invalid Relational Operator when Using Aggregate Inside Subquery

Introduction

Oracle database is a powerful tool for managing and analyzing large amounts of data. However, it can be challenging to write efficient queries that meet specific requirements. In this article, we will explore the issue of ORA-00920: invalid relational operator when using aggregate inside subquery.

Understanding Oracle Subqueries

Before diving into the problem at hand, let’s take a brief look at how subqueries work in Oracle. A subquery is a query nested inside another query, which can be used to retrieve specific data or perform calculations.

Subqueries can be categorized into two types:

Correlated subquery: This type of subquery involves referencing columns from the outer query in the inner query. Correlated subqueries are necessary when we want to use aggregate functions like SUM() or MAX() in the subquery.
Non-correlated subquery: This type of subquery does not reference any columns from the outer query and can be used with simple queries.

Using Aggregate Functions Inside Subqueries

Oracle’s SQL syntax allows us to use aggregate functions like SUM(), MIN(), and MAX() inside subqueries. However, these functions cannot be used directly in the WHERE clause or HAVING clause.

Instead, we need to use a HAVING clause with aggregate functions that are allowed outside of subqueries. For example:

SELECT *
FROM my_table
WHERE SUM(column) > 10;

This is valid because we’re not referencing any columns from the outer query in this subquery.

However, when we try to use an aggregate function inside a subquery and reference columns from the outer query, Oracle throws an error:

SELECT *
FROM my_table
WHERE (SELECT COUNT(*) FROM subquery WHERE column > 10) > 5;

This is not valid because of the error ORA-00920: invalid relational operator.

Understanding the Issue

So why does this error occur?

The main reason for this error lies in how Oracle handles correlated subqueries and aggregate functions. In a correlated subquery, we’re referencing columns from the outer query that are dependent on the result of the inner query.

Oracle’s SQL syntax doesn’t allow us to use aggregate functions inside a correlated subquery because it would prevent us from using the function for all rows in the table.

A More Detailed Look

Let’s dive into the specifics. Consider this example:

SELECT *
FROM my_table CN
WHERE EXISTS (
    SELECT 't' 
    FROM Table1 CN2 
    WHERE CN2.MyColumn1  = CN.MyColumn1 
        AND CN2.MyColumn5= 0 
        AND (CN2.MyColumn2- CN2.MyColumn3+ CN2.MyColumn4) &gt;= 
            SUM(CN.MyColumn2- CN.MyColumn3 + CN.MyColumn4) * 0.5)
);

In this query, we’re using a correlated subquery to check whether the difference between CN2.MyColumn2, CN2.MyColumn3 and CN2.MyColumn4 is greater than or equal to 50% of the sum of the differences in CN.MyColumn2, CN.MyColumn3 and CN.MyColumn4.

However, this query contains an aggregate function (SUM) that cannot be used inside a correlated subquery. Therefore, Oracle throws an error.

Workaround: Using Having Clause

As shown in the original code example:

SELECT CN.MyColumn1,
       SUM(CN.MyColumn2 - CN.MyColumn3 + CN.MyColumn4),
       (CASE 
          WHEN EXISTS (
             SELECT 't' 
             FROM Table1 CN2 
             WHERE CN2.MyColumn1  = CN.MyColumn1 
               AND CN2.MyColumn5= 0 
            HAVING (CN2.MyColumn2- CN2.MyColumn3+ CN2.MyColumn4) &gt;= 
                SUM(CN.MyColumn2- CN.MyColumn3 + CN.MyColumn4) * 0.5)
          THEN 'Yes' 
          ELSE 'No' 
       END)
FROM Table1 CN 
GROUP BY CN.MyColumn1 
HAVING SUM (CN.MyColumn2- CN.MyColumn3 + CN.MyColumn4) &lt; 6;

We’ve restructured the query to move the aggregate function into a separate HAVING clause. This is valid because aggregate functions can be used in the HAVING clause.

Conclusion

In conclusion, while Oracle SQL allows us to use aggregate functions in various contexts, there are limitations when it comes to using these functions inside correlated subqueries.

To resolve this issue, we need to restructure our query and move the aggregate function into a separate HAVING clause. This ensures that the function is evaluated for all rows in the table and provides accurate results.

By understanding how Oracle handles correlated subqueries and aggregate functions, you can write more efficient and effective queries that meet your specific requirements.

Example Use Case

Suppose we have an e-commerce database with sales data. We want to retrieve a list of products that were sold at a discount greater than or equal to 50% of their original price.

Here’s how we can modify the query:

SELECT *
FROM products P
WHERE EXISTS (
    SELECT 't' 
    FROM orders O 
    WHERE O.ProductID  = P.ProductID 
        AND O.DiscountAmount &gt;= 
            (P.Price * 0.5)
);

This modified query retrieves the list of products that meet our condition using a correlated subquery.

Conclusion

In this article, we explored the issue of ORA-00920: invalid relational operator when using aggregate inside subquery in Oracle database.

We covered how Oracle SQL handles correlated subqueries and aggregate functions, why it throws an error when trying to use these functions together, and provided solutions for restructuring our query to avoid this limitation.

By understanding how these concepts work, you can write more efficient and effective queries that meet your specific requirements.

Last modified on 2024-01-11