Change Null Values In A Table To The Previous Value - Leetcode Solution

Problem Statement:

Given a table named 'stages' containing two columns 'stage' and 'day'. The table contains information about the users who completed a particular stage of a game on a given day.

However, there are some null values in the 'day' column, which represent the days on which the particular user did not complete the stage. You are required to replace these null values with the previous 'day' value for each particular 'stage'.

Write a SQL query to solve the problem.

Solution:

In order to solve the problem, we need to identify the previous 'day' value for a particular 'stage' for each row where the 'day' value is null. We can then update the null values with their respective previous 'day' values.

To achieve this, we need to use two concepts: subqueries and self-joins.

First, we need to create a subquery that will return the previous 'day' value for a particular 'stage' for each row.

The subquery will look like this:

SELECT s1.stage, MAX(s2.day) AS prev_day
FROM stages s1
JOIN stages s2 ON s1.stage = s2.stage AND s1.day > s2.day
GROUP BY s1.stage, s1.day;

In this subquery, we are joining the 'stages' table with itself using the 'stage' column and comparing the 'day' of each row to the 'day' of all the previous rows for that particular 'stage'. We then group the results by 'stage' and 'day' and select the maximum 'day' value as the previous 'day' value.

Next, we need to use this subquery to update the null values in the 'day' column of the 'stages' table.

The update query will look like this:

UPDATE stages
SET day = (
    SELECT prev_day
    FROM (
        SELECT s1.stage, MAX(s2.day) AS prev_day
        FROM stages s1
        JOIN stages s2 ON s1.stage = s2.stage AND s1.day > s2.day
        GROUP BY s1.stage, s1.day
    ) t
    WHERE t.stage = stages.stage
    AND t.prev_day IS NOT NULL
    AND stages.day IS NULL
)
WHERE day IS NULL;

In this update query, we are using the subquery we created earlier to update the null values in the 'day' column. We select the 'prev_day' value from the subquery where the 'stage' value in the subquery matches the 'stage' value in the 'stages' table and the 'prev_day' value is not null. We then update the null values in the 'day' column with this value.

Note: It is important to validate the 'prev_day' value we are using to update the null values. we are selecting the 'prev_day' value from the subquery only if it is not null. This avoids replacing null values with a potentially incorrect value.

Overall, the solution involves using subqueries and self-joins to identify the previous 'day' value for each 'stage', and then using an update query to replace the null values with the previous 'day' values.

1