Flattening rows into a single row based on rules?

I have a result set that looks something like this:

I want to ‘flatten’ it so that it outputs this:

Is this possible?

>Solution :

This is a gaps and islands problem. One approach uses the difference in row numbers method:

WITH cte AS (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY customer ORDER BY date_from) rn1,
              ROW_NUMBER() OVER (PARTITION BY customer, flag ORDER BY date_from) rn2
    FROM yourTable
)

SELECT
    customer,
    flag,
    MIN(date_from) AS date_from,
    MAX(date_to) AS date_to
FROM cte
GROUP BY
    customer,
    flag,
    rn1 - rn2
ORDER BY
    MIN(date_from);