Question

How to select the first element of each day in a month with mysql query ?

I have table with offers – startdate, so i can check for each day,month,year i’m getting the element but, i’m wondering how to get only the first element in each day of some month ?

Answer 1

Assume the following

• Table is called mytable
• Table has id as primary key
• Table has dt as datatime
• You want the first id of everyday in February 2012

Try this:

SELECT B.id FROM
(
    SELECT DATE(dt) date_dt,MIN(dt) dt
    FROM mytable
    WHERE dt >= '2012-02-01 00:00:00'
    AND   dt < '2012-03-01 00:00:00'
    GROUP BY DATE(dt)
) A
LEFT JOIN mytable B USING (dt);

If any dt has multiple B.id values try this:

SELECT dt,MIN(id) id
(
    SELECT B.id,B.dt FROM
    (
        SELECT DATE(dt) date_dt,MIN(dt) dt
        FROM mytable
        WHERE dt >= '2012-02-01 00:00:00'
        AND   dt < '2012-03-01 00:00:00'
        GROUP BY DATE(dt)
    ) A
    LEFT JOIN mytable B USING (dt)
) AA GROUP BY dt;