Select the first element in each day of the month - mysql

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;

Answer 2

Assuming startdate is a DATETIME type, and the earliest entry is the one with the earliest DATETIME value, for March, 2012:
SELECT DISTINCT *
FROM tbl t1
LEFT JOIN tbl t2
ON (t2.startdate BETWEEN '2012-02-01 00:00:00' AND '2012-02-29 23:59:59')
AND t2.startdate < t1.startdate
WHERE (t1.startdate BETWEEN '2012-02-01 00:00:00' AND '2012-02-29 23:59:59')
AND t2.startdate IS NULL
If there are no duplicate dates, then you don't need the DISTINCT.
This query works by joining with any earlier record for the same month, so if nothing was joined, it's the earliest, through process of elimination.
This technique is explained in detail in the book SQL Antipatterns.
This could also be solved with subqueries, but this type of JOIN is supposed to be easier to optimize by MySQL than subqueries, which often negate the use of indexes.

Answer 3

without knowing the exact structure of your table something like this should work:
SELECT MIN(offerId) FROM offers WHERE startdate <= '2012-03-06' AND startdate >= '2012-02-06' GROUP BY date(startdate)

Answer 4

It sounds like you are trying to do something like the following:
SELECT col_1, date_col, col_3 FROM tbl
WHERE
date_col = ( SELECT min(date_col) FROM tbl
WHERE
year(date_col) = 2006 AND
month(date_col) = 02
);
This can also be used to find the max( date_col ) . Hope this helps.

Answer 5

Just to offer a different way to skin this cat (much easier in SQL Server for once actually)
SELECT
t0.offerId
FROM
offers AS t0 LEFT JOIN
offers AS t1 ON t0.offerId = t1.offerId AND t1.startDate > t0.startDate AND
(t0.startDate BETWEEN '2012-02-01' AND '2012-03-01') AND
(t1.startDate BETWEEN '2012-02-01' AND '2012-03-01')
WHERE
t1.col1 IS NULL;
If you have multiple rows with the same exact time you will get multiple values returned, which you can weed out in your application logic or with a sub-query. BTW this is called a groupwise minimum/maximum.