Subquery with Count and Sum: A Deep Dive into Oracle SQL

Introduction

When working with Oracle SQL, it’s not uncommon to encounter queries that involve multiple subqueries. In this article, we’ll explore a specific scenario where a user is trying to subtract the count of records from one table from the sum of records in another table using a subquery. We’ll delve into the issue, provide an explanation for why it doesn’t work, and offer a solution using Common Table Expressions (CTEs).

The Original Query

The original query is as follows:

SELECT SUM(MyColumn1) - 
  (SELECT COUNT(*) FROM MOL.Table2 e WHERE e.MyColumn2=t.Column4)
FROM MOL.Table1 t
WHERE t.Column3=SomValue;

This query attempts to calculate the sum of MyColumn1 in Table1 and then subtract the count of records in Table2 where the value in MyColumn2 matches the value in t.Column4. However, it throws an error with the message “ORA-00937: not a single-group group function.”

Understanding the Error

The error ORA-00937 is raised when Oracle SQL encounters an attempt to use a single-group aggregate function (such as SUM or COUNT) outside of a GROUP BY clause. In this case, the subquery (SELECT COUNT(*) FROM MOL.Table2 e WHERE e.MyColumn2=t.Column4) is attempting to count the number of records that match the condition. However, since it’s not part of a GROUP BY clause, Oracle assumes it should be treated as a single-group aggregate function.

To resolve this issue, we need to restructure the query to group the results correctly or use an alternative approach like Common Table Expressions (CTEs).

Using Common Table Expressions (CTEs)

A CTE is a temporary result set that’s defined within a SQL statement. We can use it to break down complex queries into smaller, more manageable parts.

Here’s an example query using CTEs:

WITH 
  tsum AS (SELECT SUM(e.sal) csum FROM emp e),
  tcnt AS (SELECT COUNT(*) ccnt FROM dept d WHERE d.deptno <= 30)
SELECT csum - ccnt
FROM tsum CROSS JOIN tcnt;

In this example, we’ve created two CTEs: tsum and tcnt. The first one calculates the sum of salaries in the emp table, while the second one counts the number of departments with a department number less than or equal to 30. We then join these two CTEs together using a CROSS JOIN, which allows us to combine each row from tsum with every row from tcnt. Finally, we select the difference between the sum and count.

How CTEs Help

CTEs help in several ways:

• They allow you to restructure complex queries into smaller, more manageable parts.
• They enable you to use recursive queries, which are useful for solving hierarchical problems.
• They make your code easier to read by breaking down long sequences of operations.

Best Practices

When working with CTEs, keep the following best practices in mind:

• Use meaningful aliases for your CTEs and tables.
• Keep your CTE definitions concise and focused on a single task.
• Avoid using recursive CTEs unless absolutely necessary.
• Test your queries thoroughly to ensure they produce the expected results.

Conclusion

In this article, we explored how to subtract the count of records from one table from the sum of records in another table using a subquery. We discussed why traditional subqueries don’t work and provided an alternative solution using Common Table Expressions (CTEs). By understanding CTEs and following best practices, you can write more efficient and readable SQL queries that solve complex problems.

Last modified on 2023-09-05