Question

Possible Duplicate:
SQL: finding longest date gap

I’ve got a table of below structure:

| LearnerId | FirstName | LastName |  EnrollmentDate  | CompletionDate |

I need to get the longest period which the school has gone without enrolling or completing learners, using a SQL query. How do I do this?

Answer 1

I’ll assume you have access to CTEs and ROW_NUMBER().

First, you need an ordered list of the dates. That is, not two columns, but one. Then you can compare one date to the immediate next date quite simply.

As you have the data in two columns, creating this one ordered list will be relatively expensive. I hope for your sake that you don’t have a huge volume of data.

WITH
  all_dates
AS
(
  SELECT EnrolmentDate  AS event_date FROM yourTable GROUP BY EnrolmentDate
  UNION
  SELECT CompletionDate AS event_date FROM yourTable GROUP BY CompletionDate
)
,
  sequenced_dates
AS
(
  SELECT
    ROW_NUMBER() OVER (ORDER BY event_date) AS id,
    event_date
  FROM
    all_dates
)
SELECT
  MAX(DATEDIFF(DAY, first_event.event_date, second_event.event_date)) AS duration
FROM
  sequenced_dates    AS first_event
INNER JOIN
  sequenced_dates    AS second_event
    ON first_event.id = second_event.id - 1