Understanding Gaps and Islands in Oracle SQL

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In this article, we’ll explore a common problem in Oracle SQL known as “gaps and islands.” This issue arises when you have data that has missing or duplicate entries within a specific column. In this case, the Values column is used to identify gaps and islands.

The Problem Statement

The provided question presents a scenario where we need to compare values in the next row and update the current row if they are the same. However, the data has gaps and islands, making it challenging to achieve this goal.

For example, consider two rows with different Values columns but the same Start_of_week and End_of_week. We want to update the End_of_week column for these rows based on the next row’s End_of_week.

Understanding Oracle SQL Row Number Functions

To solve this problem, we need to use Oracle SQL row number functions. The ROW_NUMBER() function assigns a unique number to each row within a result set based on the order of the rows.

We’ll use two row numbers in our solution:

1. seqnum: assigns a unique number to each row within the same group (i.e., Name and week_start).
2. seqnum_2: assigns a unique number to each row within the same group (i.e., Name, Value, and week_start).

The Solution

The solution involves using these row numbers to identify adjacent rows with the same values.

with
     ranked_data as (
         select t.name,
                t.values,
                t.start_of_week,
                t.end_of_week,
                row_number() over (partition by name order by start_of_week) as seqnum,
                row_number() over (partition by name, values order by start_of_week) as seqnum_2
         from your_table_name t
     ),
     matched_rows as (
         select seqnum_2, end_of_week
         from ranked_data
         group by seqnum_2
         having count(*) > 1
     )
select r.name,
       r.values,
       r.start_of_week,
       max(m.end_of_week) over (partition by r.name order by r.seqnum_2)
from ranked_data r
left join matched_rows m on r.seqnum_2 = m.seqnum_2 and r.seqnum < m.seqnum
group by r.name, r.values, r.start_of_week, r.seqnum_2;

Explanation

Let’s break down the solution:

1. We first use a common table expression (CTE) to assign row numbers to each row within the same group.
2. The matched_rows CTE groups rows with the same values by their sequence number.
3. In the main query, we left join the ranked_data CTE with the matched_rows CTE on the matching conditions (seqnum_2 and seqnum).
4. We use a window function (in this case, max) to select the maximum end_of_week value for each row based on its sequence number.

Example Use Case

Suppose we have the following table:

The solution will return:

The end_of_week column for John has been updated to reflect the value from his next row (04-Jan-20), and the same has happened for Jane.

Conclusion

In this article, we explored a common problem in Oracle SQL known as “gaps and islands.” We used row number functions to identify adjacent rows with the same values and update the end_of_week column accordingly. The provided solution demonstrates how to tackle such problems using Oracle SQL.

Last modified on 2024-05-22