Calculating a Percent with a Filter on Text Dates

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Summary

The issue at hand is calculating a percentage using SQLite with a filter on text dates. The expected result is 31.39, but the actual result is 318.58 or 0.0 when attempting to round to two decimal places. The key takeaway is that the calculation involves integer division, which is the root cause of the problem.

Root Cause

The root cause of the issue is:

  • Integer division: When dividing two integers in SQLite, the result is an integer, which is then multiplied by 100.0, resulting in an incorrect percentage.
  • Text dates: The dates are stored as text, which can lead to issues when using the BETWEEN operator, although this is not the primary cause of the problem.

Why This Happens in Real Systems

This issue occurs in real systems because:

  • Implicit type conversions: SQLite performs implicit type conversions, which can lead to unexpected results, especially when dealing with integers and floating-point numbers.
  • Lack of explicit type casting: Failing to explicitly cast the counts to floating-point numbers can result in integer division, leading to incorrect results.

Real-World Impact

The real-world impact of this issue is:

  • Inaccurate percentages: Calculating incorrect percentages can have significant consequences in various applications, such as data analysis, business intelligence, and decision-making.
  • Loss of precision: Rounding incorrect percentages to two decimal places can mask the issue, but it can still lead to incorrect conclusions and decisions.

Example or Code

SELECT ROUND(
  (100.0 * (SELECT COUNT(*) FROM x WHERE date BETWEEN '2020-01-01' AND '2020-12-31') 
  / (SELECT CAST(COUNT(*) AS REAL) FROM x)), 
  2
)

How Senior Engineers Fix It

Senior engineers fix this issue by:

  • Explicitly casting counts to floating-point numbers: Using the CAST function to convert the counts to REAL or FLOAT ensures that the division operation returns a floating-point number.
  • Using the correct order of operations: Ensuring that the multiplication by 100.0 is performed after the division operation avoids integer division and produces the correct result.

Why Juniors Miss It

Juniors may miss this issue because:

  • Lack of understanding of implicit type conversions: Failing to recognize how SQLite performs implicit type conversions can lead to unexpected results.
  • Insufficient attention to the order of operations: Not carefully considering the order of operations can result in incorrect calculations and unexpected results.