MariaDB: Delete Rows Not Containing the Maximum Value in a Column (Grouped by Another Column)

2. JOIN: This clause is key for identifying the rows to delete. It's not used for joining tables in this case.

   • We create a temporary table by joining the original table with itself. This temporary table is used for comparison.
   • We use the ON clause to specify the join condition. In this case, it ensures both rows have the same value in the group_column.

4. value_column, MAX(value_column) AS max_value:

DELETE t1
FROM your_table t1
INNER JOIN (
  SELECT group_column, MAX(value_column) AS max_value
  FROM your_table
  GROUP BY group_column
) t2
ON t1.group_column = t2.group_column
WHERE t1.value_column <> t2.max_value;

Explanation:

1. DELETE t1: This specifies we want to delete rows from the table named your_table, aliased as t1.
2. INNER JOIN: This joins your_table with a temporary result set created by the subquery.
3. Subquery:
   • SELECT group_column, MAX(value_column) AS max_value: This part calculates the maximum value of value_column for each group defined by group_column.
   • FROM your_table: The subquery selects data from the same table (your_table).
   • GROUP BY group_column: This groups the rows by the group_column.
4. ON clause: This defines the join condition. It ensures rows in t1 and the subquery result (t2) have the same value in group_column.
5. WHERE clause: This filters the joined data. It keeps only rows where value_column in t1 is not equal to the max_value (maximum value within the group) from t2.

This method uses a subquery with NOT EXISTS to check if a row's value exists as the maximum value within its group.

Here's the code:

DELETE FROM your_table t1
WHERE NOT EXISTS (
  SELECT 1
  FROM your_table t2
  WHERE t1.group_column = t2.group_column
    AND t2.value_column > t1.value_column
);

1. DELETE: Similar to the previous example, it specifies deletion from your_table.
2. WHERE NOT EXISTS: This clause checks if a subquery doesn't return any rows.
3. Subquery:
   • It selects a constant value (1) to simply check for existence, not retrieve data.
   • It searches for rows in your_table (aliased as t2) where:
     • group_column matches the current row (t1).
     • value_column in t2 is greater than the current row's value_column (t1).
4. Essentially, if a row in the subquery exists (meaning another value is greater within the group), the row from the main table (t1) isn't deleted.

Using EXISTS with a correlated subquery:

This method utilizes a correlated subquery with EXISTS to compare the current row's value to the maximum within its group.

DELETE FROM your_table
WHERE NOT EXISTS (
  SELECT *
  FROM your_table AS t2
  WHERE t1.group_column = t2.group_column
    AND t2.value_column > t1.value_column
);

1. DELETE: Similar to previous examples.
2. WHERE NOT EXISTS: This clause checks for the absence of rows in the subquery.
3. Subquery:
   • It selects all columns (*) from your_table aliased as t2.
4. The logic is similar to the previous approach. If a row exists with a higher value within the group, the current row isn't deleted.

Choosing the right method:

• The method using JOIN might be more efficient for complex filtering conditions within the subquery.
• The methods using NOT EXISTS can be simpler to write for basic scenarios.
• Consider the complexity of your specific query and choose the method that best suits your needs.