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Should you choose the MONEY or DECIMAL(x,y) datatypes in SQL Server?

When it comes to storing currency values in SQL Server, you generally have two main options: using the MONEY (or SMALLMONEY) data type, or using a scaled numeric type like DECIMAL(x, y). Although MONEY might sound like the intuitive choice for financial data, it comes with certain quirks. Below is a comparison to help you decide which is best suited for your use case.

1. Understanding Each Data Type

1.1 MONEY

  • Fixed Precision: Internally stored as an integer scaled by 10,000.
  • Display Precision: Up to four decimal places.
  • Storage: 8 bytes for MONEY; 4 bytes for SMALLMONEY.
  • Potential Round-Off Issues: Operations on MONEY can lead to rounding anomalies because of its internal scaling factor.

1.2 DECIMAL(x, y)

  • Variable Precision: x defines the total number of digits, while y defines the number of digits to the right of the decimal point.
  • Storage: The storage size depends on the specified precision (x).
  • More Control: Offers fine-grained control over precision and scale, making it ideal for financial calculations requiring exact arithmetic.

2. Potential Pitfalls with MONEY

  1. Rounding Errors: When performing arithmetic, MONEY can introduce subtle rounding discrepancies due to its internal representation.
  2. Limited Decimal Places: If you need more than four decimal places (common in certain financial calculations or foreign currencies), MONEY is insufficient.
  3. Obsolete or Discouraged: Many seasoned DBAs avoid MONEY in new designs, as Microsoft recommends DECIMAL for precise calculations.

3. Why DECIMAL(x, y) Is Often Preferred

  • Precision and Scale: You can define exactly how many digits you want to store and how many should be after the decimal point.
  • Standard Calculations: Arithmetic on DECIMAL typically follows expected rules without hidden scaling issues, making it more reliable for financial and scientific data.
  • Future-Proofing: If business requirements change (e.g., more decimals, different currency rules), you can adjust the definition of DECIMAL(x, y) more easily.

4. Performance Considerations

  • Storage Size: For smaller precision, MONEY uses 8 bytes, while DECIMAL might use slightly more or less depending on x and y. In most real-world scenarios, the difference in storage is negligible.
  • Arithmetic Speed: While MONEY may offer slightly faster math in certain cases due to integer-based operations, it’s rarely a bottleneck in modern systems. Correctness and accuracy often outweigh micro-optimizations.

5. Best Practice Recommendations

  1. Use DECIMAL(x, y) for Currency: If you need reliable calculations and flexible precision, DECIMAL is typically the safer choice.
  2. Avoid MONEY for Complex Operations: Simple sums and subtractions might be fine, but multiplication and division can yield surprises.
  3. Choose Sensible Precision: For currency, a definition like DECIMAL(19, 4) is common (supporting up to 15 digits to the left and 4 decimals).

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Conclusion

While MONEY might look convenient for monetary columns, its limited precision and rounding behavior can introduce subtle inaccuracies. DECIMAL(x, y) is generally the recommended approach for storing and calculating currency values in SQL Server, as it grants finer control over precision, delivers consistent arithmetic, and aligns better with modern database design best practices.

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