Getting Last Value from a Certain Group (Oracle)

In this article, we will explore how to get the latest value from a certain group in Oracle SQL. This can be achieved using window functions, which allow us to perform calculations across rows that are correlated with each other.

Introduction to Window Functions

Window functions are a type of aggregate function that allows you to perform calculations on a set of rows that are related to each other. They provide a way to analyze data by dividing it into groups and applying calculations to those groups.

In Oracle, window functions were introduced in version 11g, and they have since become a powerful tool for analyzing and manipulating data.

The Problem

Suppose we have a table with the following structure:

We want to get the latest value from group ‘A’, which is currently stored in the Latest ID from Group A (Ordered by Recent Date) column.

The Solution

One way to achieve this is using a window function called KEEP. The KEEP clause allows us to specify how we want to handle ties, i.e., when multiple rows have the same value.

Here’s an example query that uses the KEEP clause to get the latest value from group ‘A’:

with demo (somedate, somegroup, id) as (
  select date '2018-01-11', 'A', 1 from dual union all
    select date '2018-01-12', 'A', 2 from dual union all
    select date '2018-01-13', 'B', 3 from dual union all
    select date '2018-01-14', 'B', 4 from dual union all
    select date '2018-01-15', 'A', 5 from dual -- example from comments
)
select somedate, somegroup, id,
       (select max(id) keep (dense_rank last order by somedate)
          from demo
          where somegroup = 'A') as last_a
from demo;

In this query, we use the KEEP clause to specify that we want to get the maximum value of id for group ‘A’ when ordering by date. The DENSE_RANK function assigns a rank to each row within each group based on the specified column (in this case, date). When there are ties, the LAST keyword specifies which tiebreaker we want to use.

Explanation

The query works as follows:

1. We create a temporary view demo with three columns: somedate, somegroup, and id. We populate this table with sample data using the UNION ALL operator.
2. We select all columns from the demo view, including a new column called last_a.
3. The last_a column is calculated using a subquery that uses the KEEP clause to get the maximum value of id for group ‘A’ when ordering by date.
4. The DENSE_RANK function assigns a rank to each row within each group based on the specified column (in this case, date). When there are ties, the LAST keyword specifies which tiebreaker we want to use.

Example Use Cases

Here are some example use cases for getting the latest value from a certain group:

• Getting the latest order total: Suppose you have an order table with columns for order_id, customer_id, and total. You can use a window function to get the latest order total for each customer.

with orders (order_id, customer_id, total) as ( select order_id, customer_id, total from dual union all select 2, 1, 100 from dual select 3, 1, 200 from dual select 4, 2, 50 from dual select 5, 2, 75 from dual ) select order_id, customer_id, total, (select max(total) keep (dense_rank last order by total) from orders where customer_id = ‘1’) as latest_total from orders;

*   **Getting the latest sales figures**: Suppose you have a sales table with columns for `sales_id`, `product_id`, and `date`. You can use a window function to get the latest sales figures for each product.
    ```markdown
with sales (sales_id, product_id, date) as (
  select sales_id, product_id, date from dual union all
  select 1, 1, '2018-01-01'
  select 2, 1, '2018-01-02'
  select 3, 2, '2018-01-03'
  select 4, 2, '2018-01-04'
)
select sales_id, product_id, date,
       (select max(date) keep (dense_rank last order by date)
          from sales
          where product_id = 1) as latest_date
from sales;

• Getting the latest website traffic: Suppose you have a website log table with columns for log_id, date, and page_view_count. You can use a window function to get the latest page view count for each day.

with logs (log_id, date, page_view_count) as ( select log_id, date, page_view_count from dual union all select 1, ‘2018-01-01’, 100 select 2, ‘2018-01-01’, 200 select 3, ‘2018-01-02’, 150 select 4, ‘2018-01-03’, 120 ) select log_id, date, page_view_count, (select max(page_view_count) keep (dense_rank last order by date) from logs where date = ‘2018-01-01’) as latest_page_views from logs;

These are just a few examples of how you can use window functions to get the latest values for each group. The possibilities are endless!

Last modified on 2024-09-01