检测列值的变化？

Question

This is how my table looks

page  time
x      0
x      1
x      2
y      0
y      1
x      0
x      1
x      2
x      3 
z      0
z      1
z      2
z      3

I am plotting a graph based on values of x,y,z.

So My graph looks like--

The height of each bar is determined by peak value of respective page.

I tried something like this

`$query="select `page`,`time` from `table` where session_id="$session_id" GroupBy ...how do i go about this.";

but it gives me only 3 bars.But I want the no. of bar depending on the switch (switch from peak value to 0.In above table there are 4 such switch and thus there are 4 bars in my graph.)

I hope I am clear with my question. I am not able figure out the query.

UPDATE--> I am developing a tracker which basically keeps track of all the links clicked by a user and time spend on it.So the switch can be from X to Y to X to Z..It can be random.Thats why i am confused.

UPDATE 2: suppose if I click a link that redirects me to page x when I have already spent time==2 at the very same page x...then only max(time) is returned,I need the previous max time on X + current max time on X. In simple words,If i visit the same page again..I dont want the last recorded time to get suppressed if if spend more time on it comparitively,Instead I want to add up the time..

Answer 1

The problem is, that you want to group based on a specific order of the columns, not column page.

Here's a workaround:

CREATE TABLE Table1
    (id int auto_increment, `page` varchar(1), `time` int, primary key (id))
;

INSERT INTO Table1
    (`page`, `time`)
VALUES
    ('x', 0),
    ('x', 1),
    ('x', 2),
    ('y', 0),
    ('y', 1),
    ('x', 0),
    ('x', 1),
    ('x', 2),
    ('x', 3),
    ('z', 0),
    ('z', 1),
    ('z', 2),
    ('z', 3)
;


select page, max(time) from (
select 
t.*
, IF(@prev != page, @counter := @counter + 1, @counter) as my_groups
, @prev:=page
from
Table1 t
, (select @prev:=NULL, @counter:=0) v 
order by id
) sq
group by my_groups

See it working live here in an sqlfiddle .

Note that I introduced the column id . It's just a column that helps to guarantee an order. In a database there's no first or last row. The order in which you inserted the rows is not the guaranteed order when you simply select from the table.

Explanatation:

Here

Table1 t
, (select @prev:=NULL, @counter:=0) v 

we initialize our variables inside the query. It's the same as writing

SET @prev:=NULL;
SET @counter:=0;
SELECT ... FROM Table1 ORDER BY whatever...

Now in the SELECT part of the subquery

, IF(@prev != page, @counter := @counter + 1, @counter) as my_groups
, @prev:=page

these 2 rows are important in the order they are written. The second row assigns the value of the read row to the variable @prev. In the first row we check if @prev is different from the row which is read by SELECT. If yes, we increase our counter variable @counter by 1, if no we leave it as it is.

The subquery (a bit modified to make it more clear what happens)

select 
IF(@prev != page, @counter := @counter + 1, @counter) as my_groups
, page
, @prev
, @prev:=page
, time
from
Table1 t
, (select @prev:=NULL, @counter:=0) v
order by id

returns this result then:

| MY_GROUPS | PAGE |  @PREV | @PREV:=PAGE | TIME |
--------------------------------------------------
|         0 |    x | (null) |           x |    0 |
|         0 |    x |      x |           x |    1 |
|         0 |    x |      x |           x |    2 |
|         1 |    y |      x |           y |    0 |
|         1 |    y |      y |           y |    1 |
|         2 |    x |      y |           x |    0 |
|         2 |    x |      x |           x |    1 |
|         2 |    x |      x |           x |    2 |
|         2 |    x |      x |           x |    3 |
|         3 |    z |      x |           z |    0 |
|         3 |    z |      z |           z |    1 |
|         3 |    z |      z |           z |    2 |
|         3 |    z |      z |           z |    3 |

The outer query is simple then. We group by the column my_groups (we aliased this row with it)

IF(@prev != page, @counter := @counter + 1, @counter) as my_groups

and for each group we return the page and the maximum time.

Hope it's all clear now. Feel free to ask, though, if that's not the case :)

Answer 2

you can do something like:

select page, max(time) from table where session_id = ...
group by page

but according to your table structure above, it will give only 3 values since 1st and 3rd x would be treated as same. This is because of the way your table is defined. So you will get 3 graphs and not 4.