How to Efficiently Query Multiple MySQL Tables to Find Discrepancies While Handling NULL Values?

MySQL "NOT IN" Querying Multiple Tables

The given MySQL query seeks to identify discrepancies between data in the Grade and Evaluation tables by excluding rows from Grade that are present in Evaluation. However, the original query fails to retrieve any results when the specified name ("JOHN") is not found in Evaluation.

Understanding the Issue with "NOT IN"

The "NOT IN" operator, as used in the original query, is problematic if any column involved might contain NULL values. In MySQL, NULL represents an unknown or missing value, and its interaction with certain operators can lead to unexpected results.

Solution: Avoiding "NOT IN"

To address this issue, it's recommended to avoid using "NOT IN" when dealing with columns that may contain NULLs. Instead, consider using alternative approaches:

• EXPLICIT JOINS: Instead of using the WHERE clause for joining tables, explicitly specify the joins using JOIN statements. This allows for more precise and predictable results.
• NOT EXISTS SUBQUERY: Check for the existence of rows in one table that do not exist in another table using a nested NOT EXISTS subquery.

Example: Using NOT EXISTS

The following modified query uses NOT EXISTS to achieve the desired results:

SELECT
  G.ID_Courses,
  C.ID_Courses,
  G.NAME,
  C.NAME,
  G.ID_Courses,
  E.NAME,
  E.Year,
  G.Year
FROM Grade AS G
INNER JOIN Course AS C
  ON G.ID_Courses = C.ID_Courses
LEFT JOIN Evaluation AS E
  ON G.NAME = E.NAME AND G.Year = E.Year
WHERE
  NOT EXISTS(
    SELECT
      1
    FROM Evaluation
    WHERE
      NAME = G.NAME AND Year = G.Year
  )
  AND G.NAME = 'JOHN'
  AND G.Year = 1
GROUP BY
  G.ID_Courses;

By using NOT EXISTS, this query ensures that rows from Grade that do not have corresponding rows in Evaluation will still be included in the output.

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