How to Efficiently Count Consecutive Value Occurrences in a SQL Table?

Counting Consecutive Occurrences of Values in a Table

In a database with a table containing a column of values, determining the count of consecutive occurrences of each value can be a useful task for various data analysis scenarios. This question explores the challenge of achieving this count using efficient SQL techniques.

The provided data consists of a table with an Id column and a Name column containing values 'A', 'B', and 'C'. The objective is to count the consecutive occurrences of these values in the Name column and present the results as follows:

+------+------------+
| Name | Repetition |
+------+------------+
| A    |          2 |
| B    |          4 |
| C    |          1 |
| B    |          2 |
+------+------------+

One approach attempted was using the COUNT() function over a partition by Name, but this yielded incorrect results. To address this, an alternative method leveraging row numbers can be employed.

The following code snippet presents an improved solution:

select name, count(*)
from (
    select t.*,
           (
               row_number() over (order by id) -
               row_number() over (partition by name order by id)
           ) as grp
    from t
) t
group by grp, name;

This approach utilizes the difference between two row numbers to identify consecutive occurrences. By subtracting the row number within each group from the overall row number, a unique grouping identifier is created for each consecutive value sequence. The COUNT() function is then applied to these groups, providing the desired count for each consecutive occurrence of values.

The above is the detailed content of How to Efficiently Count Consecutive Value Occurrences in a SQL Table?. For more information, please follow other related articles on the PHP Chinese website!