Get average time between times in SQL

I'm really clueless as how开发者_开发问答 to do this.

Say my table looks like so:

MESSAGES
- id
- date_posted
- content

How do I get the average time between postings? (date_posted)

SELECT  AVG(period)
FROM    (
        SELECT  TIME_TO_SEC(TIMEDIFF(@date_posted, date_posted)) AS period,
                @date_posted := date_posted
        FROM    (
                SELECT @date_posted := NULL
                ) vars,
                messages
        ORDER BY
                date_posted
        ) q

Try this:

SELECT AVG(DATEDIFF(s, M2.Date_Posted, M1.Date_Posted)) as AverageSeconds
   FROM Messages M1 INNER JOIN Messages M2 ON M1.Id = M2.Id+1

Sorry this is MS-SQL solution not mySQL - apologies only just realised that is what you were after. This may be of use to someone else.

In response to comment below - in SQL Server 2005 or 2008 - the following would be a better solution because it makes no assumption about sequential or in-fact integer Primary Keys as it uses Row_Number to generate a unique, sequential key:

SELECT AVG(DATEDIFF(s, M2.Date_Posted, M1.Date_Posted)) as AverageSeconds
    FROM (SELECT ROW_NUMBER() OVER(ORDER BY Date_Posted ASC) as Id, Date_Posted FROM 
            Messages) M1
    INNER JOIN (SELECT ROW_NUMBER() OVER(ORDER BY Date_Posted ASC) as Id, Date_Posted 
            FROM Messages) M2 ON M1.Id = M2.Id+1