日期间隔错误

Question

I have a table which has a column called created DATETIME , When adding entries It works fine, but when It gets to Date Interval, It keeps on duplicating new entries. I'm trying to get Today's and Yesterday's entries. Today's works fine, but for Yesterday's, It also puts on Today's contents in query results which Is not what I want.

SELECT * FROM tab WHERE created > DATE_SUB(NOW(), INTERVAL $num DAY) ORDER BY created DESC LIMIT 9;

$num Is 1 for Today's entries, and It's 2 for Yesterday's. So basically an entry which Is created today, Is getting duplicated on Yesterday's query results.

Answer 1

You are getting the results you requested from the database. Namely any record that is greater than today minus however many days you put in.

The reason you get 0 records when you try @KenWhite's suggested of changing your > to = is because your field is DATETIME, so subtracting exactly 24 hours from NOW() yields the same exact time yesterday and you probably don't have a record that was written precisely at this time yesterday. Right?

What you'll have to do is test for records between two dates to get you want. Instead of NOW(), switch to CURDATE(), this way you can be assured you'll get every record for the datetime range you are looking for.

SELECT *
FROM tab
WHERE 
  created BETWEEN DATE_SUB(CURDATE(), INTERVAL $num DAY) AND DATE_SUB(CURDATE(), INTERVAL $num - 1 DAY) 
ORDER BY created DESC LIMIT 9;

You can check out a SQLFiddle of this here: http://sqlfiddle.com/#!2/19d9b/12

Answer 2

With datetime/timestamp values, similar to floats, always use ranges with closed beginnings and open endings. So use '>=' and '<'.

For example to use the data of a single day:

SELECT ... FROM tab 
WHERE created >= CURDATE() - INTERVAL @num:=? DAY
  AND created <  CURDATE() - INTERVAL @num    DAY + INTERVAL 1 DAY
ORDER BY created DESC
LIMIT 9
;

With MySQL, generally prefer the timestamp type over the datetime type, as datetime doesn't carry timezone information.

Alternatives:

created_at timestamp NULL DEFAULT NULL COMMENT 'set by application',

created_at timestamp NOT NULL DEFAULT '1970-01-01 23:00:00' COMMENT 'set by application',

dbms_row_created_at timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT 'set by DBMS',