SELECT data based on result of previous row in table - mysql

Question

I have a database of students.
CREATE TABLE classlist
(`id` int, `studentid` int, `subjectid` int, `presentid` int)
;
CREATE TABLE student
(`id` int, `name` varchar(4))
;
CREATE TABLE subject
(`id` int, `name` varchar(4))
;
CREATE TABLE classStatus
(`id` int, `name` varchar(8))
;
INSERT INTO classlist
(`id`, `studentid`, `subjectid`, `presentid`)
VALUES
(1, 111, 1, 1),
(2, 222, 3, 0),
(3, 333, 2, 1),
(4, 111, 4, 1),
(5, 111, 1, 0),
(6, 222, 3, 0),
(7, 333, 2, 1),
(8, 111, 4, 1),
(9, 111, 2, 0),
(10, 111, 4, 1),
(11, 111, 1, 1),
(12, 333, 3, 1),
(13, 333, 2, 1),
(14, 333, 3, 1)
;
INSERT INTO student
(`id`, `name`)
VALUES
(111, 'John'),
(222, 'Kate'),
(333, 'Matt')
;
INSERT INTO subject
(`id`, `name`)
VALUES
(1, 'MATH'),
(2, 'ENG'),
(3, 'SCI'),
(4, 'GEO')
;
INSERT INTO classStatus
(`id`, `name`)
VALUES
(0, 'Absent'),
(1, 'Present')
;
And I have a query which shows how many times they have been present or absent.
SELECT
studentid,
students.name AS NAME,
SUM(presentid = 1) AS present,
SUM(presentid = 0) AS absent
FROM classlist
INNER JOIN student as students ON classlist.studentid=students.id
GROUP BY studentid, NAME
See this fiddle below.
http://sqlfiddle.com/#!2/fe0b0/1
There seems to be a trend from looking at this sample data that after someone attends subjectid 4 they are often not coming to the next class. How can I capture this in a query. I want to ONLY show data WHERE last subjectid =4. So in my sample data rows matching my criteria would be.
(5, 111, 1, 0),
(9, 111, 2, 0),
(11, 111, 1, 1),
as these rows are all the next row of a studentid who had a subjectid=4.
My output would be
| STUDENTID | NAME | PRESENT | ABSENT|
| 111 | John | 1 | 2 |

Answer 1

To get the next class for a student, use a correlated subquery:
select cl.*,
(select min(cl2.id) from classlist cl2 where cl2.studentid = cl.studentid and cl2.id > cl.id) as nextcl
from classlist cl
Plugging this into your query example tell you you who is present and absent for the next class:
SELECT students.id, students.name AS NAME,
SUM(cl.presentid = 1) AS present, SUM(cl.presentid = 0) AS absent,
sum(clnext.presentid = 1) as presentnext
FROM (select cl.*,
(select min(cl2.id) from classlist cl2 where cl2.studentid = cl.studentid and cl2.id > cl.id) as nextcl
from classlist cl
) cl INNER JOIN
student as students
ON cl.studentid = students.id left outer join
classlist clnext
on cl.nextcl = clnext.id
GROUP BY students.id, students.NAME
Add a where cl.subjectid = 4 to get the answer for subject 4.
I fixed the query. The SQLFiddle is k.

Answer 2

A quick and dirty solution could be to get the Classlist.Id for all lines where subjectid=4 (let's call them n) then select all the lines where Id = n+1