Null Values during Query - mysql

Question

I have the below table, pretty simple.
==========================================================================
attendanceID | agentID | incurredDate | points | comment
==========================================================================
10 | vimunson | 2013-07-22 | 2 | Some Text
11 | vimunson | 2013-07-29 | 2 | Some Text
12 | vimunson | 2013-12-06 | 1 | Some Text
The his query below:
SELECT
attendanceID,
agentID,
incurredDate,
leadDate,
points,
#1F:=IF(incurredDate <= curdate() - 90
AND leadDate = NULL,
points - 1,
IF(DATEDIFF(leadDate, incurredDate) > 90,
points - 1,
points)) AS '1stFallOff',
#2F:=IF(incurredDate <= curdate() - 180
AND leadDate = NULL,
points - 2,
IF(DATEDIFF(leadDate, incurredDate) > 180,
points - 2,
#1F)) AS '2ndFallOff',
IF(#total < 0, 0, #total:=#total + #2F) AS Total,
comment,
linked
FROM
(SELECT
attendanceID,
mo.agentID,
#r AS leadDate,
(#r:=incurredDate) AS incurredDate,
comment,
points,
linked
FROM
(SELECT
m . *
FROM
(SELECT #_date = NULL, #total:=0) varaible, attendance m
ORDER by agentID , incurredDate desc) mo
where
agentID = 'vimunson'
AND (case
WHEN #_date is NULL or #_date <> incurredDate THEN #r:=NULL
ELSE NULL
END IS NULL)
AND (#_date:=incurredDate) IS NOT NULL) T
ORDER BY agentID , incurredDate
When I run the query it returns the below:
========================================================================================================================================
attendanceID | agentID | incurredDate | leadDate | points | 1stFallOff | 2ndFallOff | Total | comment
========================================================================================================================================
10 | vimunson | 2013-07-22 | NULL | 2 | 2 | 2 | 2 | Some Text
11 | vimunson | 2013-07-29 | NULL | 2 | 2 | 2 | 4 | Some Text
12 | vimunson | 2013-12-06 | NULL | 1 | 2 | 1 | 5 | Some Text
I cannot figure out why the leadDate column is `null'. I have narrowed it down to a user session. For example if I run it again with the same user session it will come back with what I want.

Answer 1

The way variables #r and #_date are passed around relies on a specific order in which certain parts of the query are evaluated. That's a risky assumption to make in a query language that is declarative rather than imperative. The more sophisticated a query optimizer is, the more unpredictable the behaviour of this query will be. A 'simple' engine might follow your intentions, another engine might adapt its behaviour as you go, for example because it uses temporary indexes to improve query performance.
In situations where you need to pass values from one row to another, it would be better to use a cursor.
http://dev.mysql.com/doc/refman/5.0/en/cursors.html
EDIT: sample code below.
I focused on column 'leadDate'; implementation of the falloff and total columns should be similar.
CREATE PROCEDURE MyProc()
BEGIN
DECLARE done int DEFAULT FALSE;
DECLARE currentAttendanceID int;
DECLARE currentAgentID, previousAgentID varchar(8);
DECLARE currentIncurredDate date;
DECLARE currentLeadDate date;
DECLARE currentPoints int;
DECLARE currentComment varchar(9);
DECLARE myCursor CURSOR FOR
SELECT attendanceID, agentID, incurredDate, points, comment
FROM attendance
ORDER BY agentID, incurredDate DESC;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
CREATE TEMPORARY TABLE myTemp (
attendanceID int,
agentID varchar(8),
incurredDate date,
leadDate date,
points int,
comment varchar(9)
) ENGINE=MEMORY;
OPEN myCursor;
SET previousAgentID := NULL;
read_loop: LOOP
FETCH myCursor INTO currentAttendanceID, currentAgentID, currentIncurredDate, currentPoints, currentComment;
IF done THEN
LEAVE read_loop;
END IF;
IF previousAgentID IS NULL OR previousAgentID <> currentAgentID THEN
SET currentLeadDate := NULL;
SET previousAgentID := currentAgentID;
END IF;
INSERT INTO myTemp VALUES (currentAttendanceID, currentAgentID, currentIncurredDate, currentLeadDate, currentPoints, currentComment);
SET currentLeadDate := currentIncurredDate;
END LOOP;
CLOSE myCursor;
SELECT *
FROM myTemp
ORDER BY agentID, incurredDate;
DROP TABLE myTemp;
END
FYC: http://sqlfiddle.com/#!2/910a3/1/0

Answer 2

Try this:
SELECT a.attendanceID, a.agentID, a.incurredDate, a.leadDate, a.points,
#1F:= (CASE WHEN incurredDate <= CURDATE()-90 AND leadDate=NULL THEN points-1
WHEN DATEDIFF(leadDate, incurredDate) > 90 THEN points-1
ELSE points
END) AS '1stFallOff',
#2F:= (CASE WHEN incurredDate <= CURDATE()-180 AND leadDate=NULL THEN points-2
WHEN DATEDIFF(leadDate, incurredDate) > 180 THEN points-2
ELSE #1F
END) AS '2ndFallOff',
(#Total:=#Total + a.points) AS Total, a.comment
FROM (SELECT a.attendanceID, a.agentID, a.incurredDate, b.incurredDate leadDate, a.points, a.comment
FROM attendance a
LEFT JOIN attendance b ON a.agentID = b.agentID AND a.incurredDate < b.incurredDate
GROUP BY a.agentID, a.incurredDate
) AS A, (SELECT #Total:=0, #1F:=0, #2F:=0) B;
Check the SQL FIDDLE DEMO
OUTPUT
| ATTENDANCEID | AGENTID | INCURREDDATE | LEADDATE | POINTS | 1STFALLOFF | 2NDFALLOFF | TOTAL | COMMENT |
|--------------|----------|---------------------------------|---------------------------------|--------|------------|------------|-------|-----------|
| 10 | vimunson | July, 22 2013 00:00:00+0000 | July, 29 2013 00:00:00+0000 | 2 | 2 | 2 | 2 | Some Text |
| 11 | vimunson | July, 29 2013 00:00:00+0000 | December, 06 2013 00:00:00+0000 | 2 | 1 | 1 | 4 | Some Text |
| 12 | vimunson | December, 06 2013 00:00:00+0000 | (null) | 1 | 1 | 1 | 5 | Some Text |

Answer 3

After reviewing multiple results I was able to come up with something that is what I expect. I have entered more data and the below syntax. I liked the idea of the cursor but it was not ideal for my use, so I did not use it. I did not want to use CASE or any JOINS since they can be complex.
http://sqlfiddle.com/#!2/2fb86/1
SELECT
attendanceID,
agentID,
incurredDate,
#ld:=(select
incurredDate
from
attendance
where
incurredDate > a.incurredDate
and agentID = a.agentID
order by incurredDate
limit 1) leadDate,
points,
#1F:=IF(incurredDate <= DATE_SUB(curdate(),
INTERVAL IF(incurredDate < '2013-12-02', 90, 60) DAY)
AND #ld <=> NULL,
points - 1,
IF(DATEDIFF(COALESCE(#ld, '1900-01-01'),
incurredDate) > IF(incurredDate < '2013-12-02', 90, 60),
points - 1,
points)) AS '1stFallOff',
#2F:=IF(incurredDate <= DATE_SUB(curdate(),
INTERVAL IF(incurredDate < '2013-12-02',
180,
120) DAY)
AND getLeadDate(incurredDate, agentID) <=> NULL,
points - 1,
IF(DATEDIFF(COALESCE(#ld, '1900-01-01'),
incurredDate) > IF(incurredDate < '2013-12-02',
180,
120),
points - 2,
#1F)) AS '2ndFallOff',
IF((#total + #2F) < 0,
0,
IF(DATE_ADD(incurredDate, INTERVAL 365 DAY) <= CURDATE(),
#total:=0,
#total:=#total + #2F)) AS Total,
comment,
linked,
DATE_ADD(incurredDate, INTERVAL 365 DAY) AS 'fallOffDate'
FROM
(SELECT #total:=0) v,
attendance a
WHERE
agentID = 'vimunson'
GROUP BY agentID , incurredDate