How to Count Consecutive Value Occurrences in a Database Column?

How to Count Consecutive Occurrences in a Table's Column

When working with data in a table, it can be useful to count the number of times that a value occurs consecutively in a specific column. For example, if a table contains a column representing the names of employees, you might want to know the number of times that the same employee name appears in a row.

Problem

Consider the following table:

create table #t (Id int, Name char)

insert into #t values
(1, 'A'),
(2, 'A'),
(3, 'B'),
(4, 'B'),
(5, 'B'),
(6, 'B'),
(7, 'C'),
(8, 'B'),
(9, 'B')

The goal is to count the number of consecutive occurrences of each value in the "Name" column. The desired output is:

Name	Repetition
A	2
B	4
C	1
B	2

Solution

One approach to solving this problem is to use the concept of row numbers and then calculate the difference between them to identify consecutive occurrences. The following query demonstrates this approach:

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;

Explanation

The subquery in the query:

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

calculates the row numbers for each row in the 't' table. The row_number() function assigns a sequential number to each row, while the partition by clause ensures that the row numbers are reset for each distinct value in the "Name" column. This results in the 'grp' column, which indicates the group of consecutive occurrences for each combination of "Name" and "Id".

The outer query then groups the results by the 'grp' column and counts the number of occurrences for each 'grp' and 'Name' combination. This provides the final result, which counts the consecutive occurrences of each value in the "Name" column.

By utilizing the difference between row numbers, this approach accurately determines the consecutive occurrences of values in the specified column and produces the desired output.

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