Question

I am having quite a frustrating problem with a stored function in MySQL.
I have a database full of session data from users connecting to a wireless hotspot. I am trying to select individual user download statistics from within a selected month. My problem is that the subquery within the function seems to ignore the mac field in the WHERE statement. Here is my code:

CREATE FUNCTION get_month_download(mo varchar(45), box int(11), mac varchar(45))   RETURNS DOUBLE
BEGIN
    DECLARE dwnld double;
    IF mo IS NULL THEN
        SET mo := CONCAT(CONCAT(YEAR(NOW()), '-', MONTH(NOW())),'-','01');
    END IF;
    SET dwnld := (
        SELECT SUM(`tx_bytes`) 
        FROM `session` 
        WHERE `assoc_time` > UNIX_TIMESTAMP(mo) 
        AND `disassoc_time` < UNIX_TIMESTAMP(DATE_ADD(mo, INTERVAL 1 MONTH))
        AND `mac` = mac
        AND `controller_id` = box
    );
    return dwnld;
END

Running this:

SELECT get_month_download('2012-09-01', '2', '00:21:5c:56:be:a3');

Returns download data for the entire table, though it is using the controller_id to filter the data.

If I run the subquery outside of the function using the same parameters, it works fine. What gives?

To be more clear, running this query:

SELECT SUM(`tx_bytes`) 
FROM `session` 
WHERE `assoc_time` > UNIX_TIMESTAMP('2012-09-01') 
AND `disassoc_time` < UNIX_TIMESTAMP(DATE_ADD('2012-09-01', INTERVAL 1 MONTH))
AND `mac` = '00:21:5c:56:be:a3'
AND `controller_id` = '2';

returns the correct download statistic for that user. Where the function returns the statistic for all users of that controller.

Answer 1

I should really reflect back to what my teachers taught me in college more often. The issue was that the variable mac, in the scope of the select statement, was seen as a field from the table, and not as my parameter from the function. So, changing the parameter name fixed the issue:

CREATE FUNCTION get_month_download(mo varchar(45), box int(11), inmac varchar(60)) RETURNS DOUBLE 
BEGIN  
    DECLARE dwnld double;
    IF mo IS NULL THEN
        SET mo := CONCAT(CONCAT(YEAR(NOW()), '-', MONTH(NOW())),'-','01');
    END IF;
    SET dwnld := (
        SELECT SUM(`tx_bytes`)   
        FROM `session`    
        WHERE `assoc_time` > UNIX_TIMESTAMP(mo)    
        AND `disassoc_time` < UNIX_TIMESTAMP(DATE_ADD(mo, INTERVAL 1 MONTH))      
        AND `controller_id` = box
        AND `mac` = inmac
    );
     return dwnld; 
END

This has been a ‘smack in the face’ moment. Thank you all for your help.