Problem Statement
Many database developers encounter unexpected discrepancies when performing calculations in SQL Server. One common issue arises when the same mathematical expression is evaluated differently. For instance, consider the following SQL Server code snippet:
DECLARE @Number1 AS DECIMAL(26,7) = 0.9009000;
DECLARE @Number2 AS DECIMAL(26,7) = 1.000000000;
DECLARE @Number3 AS DECIMAL(26,7) = 1000.00000000;
DECLARE @Result AS DECIMAL(26,7);
SET @Result = (@Number1 * @Number2) / @Number3;
SELECT @Result; -- 0.0009000
SET @Result = (@Number1 * @Number2);
SET @Result = (@Result / @Number3);
SELECT @Result; -- 0.0009009
In the first case, the output is 0.0009000, while in the second case, the output is 0.0009009. This divergence raises the question: Why are the results different when the same calculation is performed?
Explanation. Single Step Calculation
In the first approach, the entire expression (@Number1 * @Number2) / @Number3 is computed in a single step:
- SQL Server first computes the product of @Number1 and @Number2, which equals 0.9009000.
- Next, it divides that result by @Number3 (1000.00000000).
The result of this division is affected by how SQL Server handles precision and rounding for decimal operations. This might introduce slight inaccuracies, leading to the outcome of 0.0009000.
Multiple Step Calculation
In the second approach, the operations are separated into two distinct steps:
- First, the calculation @Number1 * @Number2 is executed and stored in @Result. This retains the value of 0.9009000.
- Then, the variable @Result is divided by @Number3 in a separate statement.
This step-by-step division allows SQL Server to apply different rounding and precision rules, which can sometimes yield a more accurate result of 0.0009009.
Conclusion
The difference in outputs can often be attributed to the varying treatment of precision and rounding during calculations:
- In a single-step calculation, SQL Server evaluates the entire expression at once, potentially altering precision during the process.
- In a multiple-step calculation, SQL Server retains more precision through intermediate results, leading to a different output.
Resolution
To achieve consistent results in SQL Server calculations, developers should consider controlling precision explicitly. For example, applying rounding can help standardize outcomes:
SET @Result = ROUND((@Number1 * @Number2) / @Number3, 7);
By managing precision and rounding explicitly, programmers can avoid discrepancies and ensure that their numerical calculations yield the expected results. Understanding these nuances in SQL Server can lead to more reliable and accurate database operations.