SQL: Filtering Results Based on Existence or Non-Existence of Similar Results

When working with large datasets, it’s often necessary to filter results based on certain conditions. One such condition is the existence or non-existence of similar results. In this article, we’ll explore different approaches to achieve this in SQL.

Understanding the Problem

The problem at hand involves filtering a set of rows based on whether there exist other rows with the same order number and part number, but different status values. Specifically, we want to include rows where both 0 and 1 are available, but not rows where both 0 and 2 or 1 and 2 are available.

Not Exists Clause

The first approach involves using the NOT EXISTS clause. This clause allows us to check if a subquery returns any results. In our case, we want to check if there exists another row with the same order number and part number, but different status value.

SELECT t.*
FROM t
WHERE NOT EXISTS (
    SELECT 1 
    FROM t t2 
    WHERE t2.order = t.order AND t2.part = t.part AND t2.status != 2
) AND
      EXISTS (
        SELECT 1 
        FROM t t2 
        WHERE t2.order = t.order AND t2.part = t.part AND t2.status IN (0, 1)
    ) AND
      EXISTS (
        SELECT 1 
        FROM t t2 
        WHERE t2.order = t.order AND t2.part = t.part AND t2.status IN (0, 1)
    );

However, this approach has a flaw. It checks for the existence of rows with status 1 and 0, but it also counts the row we’re checking as an existing row. This means that if we have two rows with the same order number and part number, but different status values, both rows will be returned.

Enhanced Logic

To fix this issue, we need to use a more complex logic. We can use the NOT EXISTS clause to check for the existence of rows with status 2, and then use the EXISTS clause to check for the existence of rows with status 0 and 1.

SELECT t.*
FROM t
WHERE NOT EXISTS (
    SELECT 1 
    FROM t t2 
    WHERE t2.order = t.order AND t2.part = t.part AND t2.status = 2
) AND
      EXISTS (
        SELECT 1 
        FROM t t2 
        WHERE t2.order = t.order AND t2.part = t.part AND t2.status IN (0, 1)
    ) AND
      EXISTS (
        SELECT 1 
        FROM t t2 
        WHERE t2.order = t.order AND t2.part = t.part AND t2.status IN (0, 1)
    );

This approach ensures that we only return rows where both 0 and 1 are available.

Max Status Query

Another approach involves using a subquery to check for the maximum status value. This can be useful if you have a large number of rows with different status values.

SELECT t.*
FROM (
    SELECT t.*, 
           MAX(t2.status) OVER (PARTITION BY t.order, t.part) AS max_status
    FROM t
) t
WHERE max_status = 0;

This query works by partitioning the data by order and part number, and then calculating the maximum status value for each partition. The outer query then selects rows where the maximum status value is 0.

Window Functions

Finally, we can use window functions to achieve this result. Window functions allow us to perform calculations across a set of rows that are related to the current row.

SELECT t.*
FROM (
    SELECT t.*, 
           SUM(CASE WHEN status = 2 THEN 1 ELSE 0 END) OVER (PARTITION BY order, part) AS num_status_2,
           SUM(CASE WHEN status = 0 THEN 1 ELSE 0 END) OVER (PARTITION BY order, part) AS num_status_0,
           SUM(CASE WHEN status = 1 THEN 1 ELSE 0 END) OVER (PARTITION BY order, part) AS num_status_1
    FROM t
) t
WHERE num_status_2 = 0 AND num_status_1 > 0 AND num_status_2 > 0;

This query works by partitioning the data by order and part number, and then calculating the sum of status values 2, 0, and 1 for each partition. The outer query then selects rows where both num_status_0 and num_status_1 are greater than zero.

Each of these approaches has its own advantages and disadvantages, and the choice of which one to use will depend on the specific requirements of your use case.

Last modified on 2024-08-20