Understanding the Problem and the Desired Output

The problem at hand involves creating a new column in a table that represents the count of values greater than -1 after a particular condition is met. In this case, the condition is when the value in the Status_measure column becomes -1. The desired output should be a count of how many times the value goes above -1 only after once the -1 is met for each name.

To illustrate this, let’s consider an example:

In this example, the value of -1 is met at row 3 (Name: a, Seconds: 20). After this point, all subsequent values in the Status_measure column should be counted as greater than -1. The desired output for Name ‘a’ would be:

The SQL Query

The problem is asking for a SQL query that achieves this. To solve it, we can use two steps:

1. Group the data by name and order by seconds.
2. Calculate the count of entries in each group where Status_measure is greater than -1.

However, this approach doesn’t consider the fact that once the -1 is met, all subsequent values should be counted as greater than -1. This can be achieved using a window function called ROW_NUMBER() to assign a row number to each row within each group (by name) and then use another window function called SUM() to count the number of rows where Status_measure is greater than -1 after the first occurrence of -1.

Step 1: Assign Row Numbers

The idea is to assign a row number to each row within each group (by name). This will allow us to identify the first row where Status_measure becomes -1 and then count all subsequent rows as greater than -1.

WITH assigned_row_numbers AS (
  SELECT *
    , row_number() OVER (PARTITION BY Name ORDER BY Seconds) AS row_num
  FROM your_table_name
)

Step 2: Count Rows Where Status_measure is Greater Than -1 After the First Occurrence of -1

Next, we need to count how many times Status_measure goes above -1 after the first occurrence of -1. To achieve this, we can use the SUM() window function with a condition that checks if the current row’s row_num is greater than 0 (i.e., it’s not the first row) and if Status_measure is greater than -1.

SELECT *
  , SUM(CASE WHEN Status_measure > -1 AND row_num > 0 THEN 1 ELSE 0 END) OVER(PARTITION BY Name) AS n
FROM assigned_row_numbers t

Putting it All Together

Now, let’s combine these steps into a single SQL query.

WITH assigned_row_numbers AS (
  SELECT *
    , row_number() OVER (PARTITION BY Name ORDER BY Seconds) AS row_num
  FROM your_table_name
)
SELECT *
  , SUM(CASE WHEN Status_measure > -1 AND row_num > 0 THEN 1 ELSE 0 END) OVER(PARTITION BY Name) AS n
FROM assigned_row_numbers t

Explanation

This query works as follows:

• The Common Table Expression (CTE) assigned_row_numbers assigns a row number to each row within each group (by name). This is done using the row_number() window function with PARTITION BY Name ORDER BY Seconds.
• The main query selects all columns (*) from the CTE and calculates a new column called n. In this column, it uses a CASE statement inside the SUM() window function to count how many times Status_measure is greater than -1 after the first occurrence of -1. It does this by checking if the current row’s row_num is greater than 0 (i.e., it’s not the first row) and if Status_measure is greater than -1.
• The PARTITION BY Name clause ensures that the count only considers rows within each group (by name).
• The result is a new column called n that contains the desired count.

Advice

The above SQL query provides an efficient way to solve the problem. However, if your table is very large or you need to optimize performance, consider using indexing on columns used in the WHERE, JOIN, and ORDER BY clauses.

Last modified on 2023-11-10