Understanding Oracle Rounding Behavior

In this article, we will explore the inconsistent behavior of Oracle’s ROUND function when used in conjunction with multiplication by a power of three. We’ll break down the steps involved and provide explanations for why this occurs.

Background on Oracle ROUND Function

The ROUND function in Oracle is used to round numbers to a specified number of decimal places. It has two modes: ROUND (up) and ROUND (down). By default, ROUND uses up mode, which means that if the fractional part of the number is less than or equal to 0.5, it rounds down; otherwise, it rounds up.

The Problem at Hand

We have a piece of code in an Oracle procedure that multiplies a NUMBER value by 3 and then rounds the result using ROUND. The problem arises because we need the rounded result to be exactly 1655.57.

WHEN in_tariff_length = 36 THEN round( (( (in_agreed / 100) * 80) * in_uplift ) / 100, 3 ) * 3

However, this code results in either 1655.58 or 1655.568 depending on the value of in_uplift.

Why Does This Happen?

To understand why ROUND behaves differently here, we need to delve into how it handles division by a power of three.

551.86 * 3 = 1655.58 (using ROUND (up))
551.856 * 3 = 1655.568 (using ROUND (down))

As you can see, when multiplying by 3, the rounding behavior changes because it now affects how Oracle handles the fractional part of the result.

The Role of Decimal Places

The key to understanding this issue lies in recognizing that when ROUND is used with a specified number of decimal places, it does not always provide the exact result we expect. Instead, it rounds up or down based on the default mode (up by default).

In our case, we’re rounding to 3 decimal places, which means ROUND uses up mode for fractional parts less than 0.5 and rounds them down.

The Solution: Multiply First

To get exactly 1655.57 as a result, we need to multiply first and then round. By doing so, we ensure that the multiplication by 3 is handled correctly without affecting the rounding behavior of ROUND.

WHEN in_tariff_length = 36 THEN 
  ((( (in_agreed / 100) * 80) * in_uplift ) / 100) * 3 + round((((( (in_agreed / 100) * 80) * in_uplift ) / 100) * 3 - 1655.57) / 0.01)

However, this approach is less efficient and more prone to errors.

A Better Approach: Use ROUND in a Different Context

Instead of manipulating the ROUND function directly, we can use it in a different context to achieve our goal. Let’s consider using the ROUND function with a larger number of decimal places first, then subtracting 1655.57 from the result multiplied by 3.

WHEN in_tariff_length = 36 THEN 
  round((( (in_agreed / 100) * 80) * in_uplift ) / 100, 10) * 3 - 1655.57

This approach ensures that we’re rounding to a higher number of decimal places first and then subtracting the desired amount.

Using Rounded Values

Another way to achieve our goal is by using rounded values directly instead of manipulating ROUND functions.

WHEN in_tariff_length = 36 THEN 
  round((( (in_agreed / 100) * 80) * in_uplift ) / 100, 3)

This approach avoids the rounding behavior issues altogether by simply specifying a fixed number of decimal places from the start.

Conclusion

In conclusion, understanding Oracle’s ROUND function and its interaction with multiplication by powers of three is crucial for avoiding unexpected results. By using ROUND in different contexts or employing alternative approaches like multiplying first and then rounding, we can achieve our desired outcome consistently.

Note: Throughout this article, it was mentioned that values are sent to and returned from functions as NUMBER. However, considering the context provided, these values were likely floating-point numbers due to the presence of decimal places.

Last modified on 2024-05-07