Resolving SQLite Query Issues with Date Formatting, Subject Renaming, and Pass/Fail Results

Understanding the Required Output, Data Constraints, and Initial Query Flaws

The goal is to generate a result set containing four columns: unique_id, test_date (formatted as MM/DD/YYYY), subject (renamed from "Science" to "Biology"), and result ("Passed" or "Not Passed") for students with valid test dates. The initial query attempts to achieve this but contains critical errors in SQL syntax, logic, and data handling.

Key Requirements Breakdown

1. Valid Test Dates: Only include rows where test_date is valid (non-null and properly formatted).
2. Date Formatting: Convert test_date from ISO format (YYYY-MM-DD) to MM/DD/YYYY.
3. Subject Renaming: Replace "Science" with "Biology" in the subject column.
4. Result Determination: Translate the numeric passed column (0 or 1) to "Not Passed" or "Passed".

Initial Query Deficiencies

1. Unnecessary Data Modification: The UPDATE statement permanently alters the subject column in the table, violating the requirement to project transformed data without modifying source data.
2. Incorrect String Replacement Logic: Using REPLACE(passed, "1", "Passed") misapplies the REPLACE function, which operates on string patterns within a column, not conditional value mapping.
3. Date Formatting Error: STRFTIME('%d/%m/%Y', test_date) uses a format string that produces DD/MM/YYYY instead of the required MM/DD/YYYY.
4. Ambiguous Filtering: WHERE test_date IS NOT NULL assumes that non-null dates are valid, but does not account for invalid date strings or formatting issues.
5. Column Reference Errors: Using double quotes ("1", "0") around numeric literals treats them as identifiers (column/table names), causing syntax errors.

Root Causes of Query Failures and Data Ambiguities

1. Misuse of SQL Functions and Clauses

• REPLACE() Function Misapplication:
  The REPLACE function replaces substrings within a column. For example, REPLACE(passed, "1", "Passed") attempts to replace the substring "1" in the passed column, which contains integers (0 or 1). Since integers are not strings, this operation fails. Even if passed were a string, this approach would not work for binary conditions.

• Incorrect Date Format Specifier:
  The format string %d/%m/%Y in STRFTIME produces a day-first date (e.g., 10/04/2022 for April 10, 2022), conflicting with the required MM/DD/YYYY format (04/10/2022).

2. Data Integrity and Schema Issues

• Invalid or Missing Test Dates:
  Rows with empty test_date (e.g., 1188946,Science,,) are excluded by WHERE test_date IS NOT NULL, but the filter does not handle malformed dates (e.g., "2019-02-30" would be invalid but not NULL).

• Non-Unique unique_id Entries:
  The dataset includes duplicate unique_id values (e.g., 1147005 appears twice), suggesting either multiple test attempts per student or a misnamed column (e.g., unique_id might represent test attempts, not students).

• Ambiguous "Passed" Logic:
  The requirement states, "if the student passed the test return the text ‘Passed’ or ‘Not Passed’." This phrasing is ambiguous but implies a direct mapping: 1 → "Passed", 0 → "Not Passed". The initial query fails to handle NULL passed values (e.g., 1237354,Science,,), which should likely be excluded.

3. Structural and Semantic Errors

• Unnecessary Data Modification:
  The UPDATE mock_test_results SET subject = "Biology"; statement alters all subject values to "Biology", destroying the original data. This violates the principle of non-destructive querying for reporting purposes.

• Identifier vs. String Confusion:
  Using double quotes ("Biology", "1") instead of single quotes ('Biology', '1') causes syntax errors. In SQLite, double quotes enclose identifiers (e.g., column names), while single quotes denote string literals.

Step-by-Step Solutions for Query Correction and Data Validation

1. Correcting the SELECT Statement Without Data Modification

Remove the UPDATE Statement:
Execute only a SELECT query to project transformed data without altering the source table:

SELECT 
  unique_id,
  -- Date formatting and renaming logic will go here
  -- Subject renaming and result translation will go here
FROM mock_test_results
WHERE [valid date condition];

Rename "Science" to "Biology":
Use a CASE expression or literal replacement:

SELECT 
  unique_id,
  -- ... other columns ...
  'Biology' AS subject,  -- All subjects are "Science", so hardcode "Biology"
  -- ... result logic ...
FROM mock_test_results
WHERE [conditions];

Format test_date as MM/DD/YYYY:
Use STRFTIME with the correct format specifiers:

STRFTIME('%m/%d/%Y', test_date) AS test_date

Translate passed to "Passed" or "Not Passed":
Use a CASE expression for conditional mapping:

CASE 
  WHEN passed = 1 THEN 'Passed' 
  WHEN passed = 0 THEN 'Not Passed' 
  ELSE 'Invalid'  -- Handle NULLs or unexpected values
END AS result

2. Filtering Valid Test Dates

Basic Filtering (Non-NULL Dates):
Exclude rows where test_date is NULL:

WHERE test_date IS NOT NULL

Advanced Date Validation:
To ensure test_date is a valid date, use SQLite’s date validation function (note: SQLite does not natively validate dates; invalid dates return NULL when parsed):

WHERE DATE(test_date) IS NOT NULL

3. Final Query Assembly

Combining all corrections:

SELECT 
  unique_id,
  STRFTIME('%m/%d/%Y', test_date) AS test_date,
  'Biology' AS subject,
  CASE 
    WHEN passed = 1 THEN 'Passed' 
    WHEN passed = 0 THEN 'Not Passed' 
    ELSE 'Invalid' 
  END AS result
FROM mock_test_results
WHERE test_date IS NOT NULL
  AND DATE(test_date) IS NOT NULL;

4. Handling Edge Cases and Data Ambiguities

Duplicate unique_id Entries:
If the goal is to list all test attempts (including duplicates), no action is needed. If only the latest attempt is required, use a subquery with ROW_NUMBER():

WITH ranked_results AS (
  SELECT 
    *,
    ROW_NUMBER() OVER (PARTITION BY unique_id ORDER BY test_date DESC) AS rank
  FROM mock_test_results
  WHERE test_date IS NOT NULL
)
SELECT 
  unique_id,
  STRFTIME('%m/%d/%Y', test_date) AS test_date,
  'Biology' AS subject,
  CASE 
    WHEN passed = 1 THEN 'Passed' 
    WHEN passed = 0 THEN 'Not Passed' 
  END AS result
FROM ranked_results
WHERE rank = 1;

Handling NULL passed Values:
Exclude rows where passed is NULL by adding AND passed IS NOT NULL to the WHERE clause.

5. Best Practices for Query Design

• Avoid Data Modification for Reporting: Use SELECT transformations instead of UPDATE unless persistence is explicitly required.
• Explicit Column Aliasing: Use AS to clarify computed columns (e.g., STRFTIME(...) AS test_date).
• Validate Input Data: Use DATE() or custom checks to filter invalid dates.
• Use Single Quotes for Literals: Always enclose string literals in single quotes ('Biology', 'Passed').

By addressing syntax errors, misapplied functions, and data integrity checks, the final query meets all specified requirements while preserving the original dataset.