Accumulating Non-Empty Columns with Oracle Queries: A Step-by-Step Guide to Efficient Querying.

Accumulating Non-Empty Columns with Oracle Queries

In this article, we’ll explore how to implement queries that return a column with an accumulated count of non-empty values from another column. We’ll delve into the details of Oracle queries and provide examples to illustrate the concepts.

Understanding the Problem

The problem statement involves creating a query that returns two columns: empty and count. The empty column should contain boolean values indicating whether a row is empty or not, while the count column accumulates the count of non-empty rows. The key constraint is that the results should not be affected by an ORDER BY clause.

Breaking Down the Problem

To tackle this problem, we’ll break it down into two separate queries: one for accumulating the count of non-empty columns and another for handling the ordering issue.

Query 1: Accumulating Count of Non-Empty Columns

The first query uses a combination of CASE statements and window functions to accumulate the count of non-empty rows. We’ll use Oracle’s COUNT function with an OVER clause to achieve this.

WITH dates_list AS
  (SELECT TO_DATE('01-31-2018','MM-dd-yyyy') + ROWNUM - 1 AS DAY
  FROM dual
    CONNECT BY LEVEL &lt;= (TO_DATE('03-31-2018','MM-dd-yyyy') - TO_DATE('01-31-2018','MM-dd-yyyy')+1)
  )
SELECT all_date, week_date, COUNT(CASE WHEN flag IS NOT NULL THEN 1 END) 
          OVER (PARTITION BY flag ORDER BY week_date) AS cnt
FROM (
  SELECT dates1.day AS all_date, dates2.day AS week_date, CASE WHEN dates2.day IS NULL THEN 0 ELSE 1 END AS flag
  FROM dates_list dates1
  LEFT JOIN
    (SELECT * FROM dates_list WHERE TO_CHAR(DAY,'D') NOT IN (7,1)) dates2 ON dates1.day = dates2.day
)
ORDER BY all_date;

This query first creates a temporary table dates_list that contains all non-weekend dates. It then selects the desired columns and applies a CASE statement to create a flag column (flag). The outer query uses an OVER clause with a partition by flag and order by week_date to accumulate the count of non-empty rows.

Query 2: Handling Ordering Issue

The second query addresses the ordering issue by using Oracle’s window functions. We’ll use the COUNT function with an OVER clause to create the cnt column, which accumulates the count of non-empty rows.

SELECT empty, 
       COUNT(CASE WHEN empty WHEN 'false' THEN 1 END) OVER (ORDER BY ord) AS cnt
FROM .....

This query is simpler than the first one and uses a combination of CASE statements and window functions to create the cnt column. The key difference is that it uses an OVER clause with an ORDER BY clause, which ensures that the results are not affected by the original ordering.

Improving the Query

To improve the query further, we can use Oracle’s window functions, such as SUM or AVG, to create a more accurate count of non-empty rows. For example, we can use the following query:

SELECT empty, 
       SUM(CASE WHEN empty THEN 1 ELSE 0 END) OVER (ORDER BY ord) AS cnt
FROM .....

This query uses the SUM function with an OVER clause to create a count of non-empty rows. The key difference is that it uses a simpler CASE statement, which eliminates the need for separate partitioning and ordering.

Conclusion

In this article, we explored how to implement queries that return a column with an accumulated count of non-empty values from another column. We broke down the problem into two separate queries: one for accumulating the count of non-empty columns and another for handling the ordering issue. By using Oracle’s window functions and CASE statements, we can create accurate and efficient queries to meet the requirements of our problem.

References

Last modified on 2023-11-16