mysql - Simple curdate() subtraction operation returning inconsistent results

Question

Here's my query:

SELECT cases.case_id, cases.county_id, case_name, county.county_name, date_referral,
date_flag, auto_flag, sessions.date_entry, curdate() - date_flag AS days_overdue
FROM cases
INNER JOIN county ON cases.county_id = county.county_id
LEFT JOIN sessions ON cases.case_id = sessions.case_id
WHERE date_flag <= curdate( )
AND sessions.date_entry IS NULL
AND date_flag IS NOT NULL
AND auto_flag = "0"
AND status_id = "0"
ORDER BY cases.county_id, days_overdue DESC , case_id ASC
LIMIT 0 , 30

Here's an example result set:

13-1206-B   McC v Jac   2013-08-15  91
13-0951-B   Gen v Gen   2013-08-16  90
13-1160-B   Bla v Bur   2013-08-20  86
12-3048-B   Wor v Wor   2013-08-29  77
13-0903-B   Ben v Ben   2013-09-01  5
13-0500-B   Bru v Bru   2013-09-05  1
13-1238-B   Bal v Bal   2013-09-05  1
13-0135-B   TDA v Tan   2013-09-06  0

The columns are case_id (irrelevant), case_name (irrelevant), date_flag, and the result of the equation "curdate() - date_flag" as days_overdue.

As you can see, the results are pretty inconsistent. Today is 2013-09-06 and it has not been 91 days since 2013-08-15. It has not been 77 days since 2013-08-29. But it has been 5 days since 2013-09-01, and it has been 0 days since 2013-09-06. So the latter half of the results are correct but the former...I don't even know where it's getting those numbers from.

Any ideas what's going wrong with this query?

Thanks!

Answer 1

Your problem is that you're trying to operate with dates as with normal operands via subtraction - operator. MySQL will not handle that. You should use DATEDIFF() function:

DATEDIFF(CURDATE(), date_flag) AS days_overdue

to get desired result (in days by default)