MySQL - DATE_ADD month interval

问题

I face a problem with the function DATE_ADD in MySQL.

My request looks like this :

SELECT * 
FROM mydb 
WHERE creationdate BETWEEN "2011-01-01" AND DATE_ADD("2011-01-01", INTERVAL 6 MONTH) 
GROUP BY MONTH(creationdate)

The problem is that, in the results, -I think- because June has only 30 days, the function doesn't work properly as I have the results of the first of July.

Is there a way to tell DATE_ADD to work well and take the right number of days within a month?

回答1:

DATE_ADD works just fine with different months. The problem is that you are adding six months to 2001-01-01 and July 1st is supposed to be there.

This is what you want to do:

SELECT * 
FROM mydb 
WHERE creationdate BETWEEN "2011-01-01" 
                   AND DATE_ADD("2011-01-01", INTERVAL 6 MONTH) - INTERVAL 1 DAY
GROUP BY MONTH(creationdate)

SELECT * 
FROM mydb 
WHERE creationdate >= "2011-01-01" 
AND creationdate < DATE_ADD("2011-01-01", INTERVAL 6 MONTH)
GROUP BY MONTH(creationdate)

For further learning, take a look at DATE_ADD documentation.

*edited to correct syntax

回答2:

Well, for me this is the expected result; adding six months to Jan. 1st July.

mysql> SELECT DATE_ADD( '2011-01-01', INTERVAL 6 month );
+--------------------------------------------+
| DATE_ADD( '2011-01-01', INTERVAL 6 month ) |
+--------------------------------------------+
| 2011-07-01                                 | 
+--------------------------------------------+

回答3:

BETWEEN ... AND

If expr is greater than or equal to min and expr is less than or equal to max, BETWEEN returns 1, otherwise it returns 0.

The important part here is EQUAL to max., which 1st of July is.

回答4:

Do I understand right that you assume that DATE_ADD("2011-01-01", INTERVAL 6 MONTH) should give you '2011-06-30' instead of '2011-07-01'? Of course, 2011-01-01 + 6 months is 2011-07-01. You want something like DATE_SUB(DATE_ADD("2011-01-01", INTERVAL 6 MONTH), INTERVAL 1 DAY).