How to get the max and min of a data from child table with mysql and php?

Question

Lets take an example:

I have 2 tables as below

Child table is related to parent table using P_Id.

Parent table contains one row for each case. Child table stores actions on each case and related by P_id, datetime colums records the time of action.

Now, I want to query what is the max of each case , as below

Second thing I want to get is max and min of each case in same table can it be done by only mysql or do i have to do using php also.

Result that I expect is as below:

Like this there are 1000's of rows in the table. Do i need to use python or something else to process this kind of data.

Answer 1

Use max() and min() aggregate functions and for max check if it is the same as min and return null if they are the same:

select data, min(sub_data), if(min(sub_data)=max(sub_data), null, max(sub_data)
from parent p
inner join child c on p.id=c.p_id

Answer 2

For your first thing:

SELECT
    p.data, c.sub_data, c.`datetime`
FROM parent p
JOIN child c
ON p.id = c.pid
JOIN (
    SELECT max(`datetime`) as `datetime`, pid
    FROM child
    GROUP BY pid
) c1
on p.id = c1.pid AND c.`datetime` = c1.`datetime`;

and second thing is:

SELECT
    p.data,
    cmin.sub_data as Min(sub_data),
    case when cmin.sub_data = cmax.sub_data then null else cmax.sub_data end as Min(sub_data)
FROM parent p
JOIN (
    SELECT max(`datetime`) as `maxdatetime`, min(`datetime`) as `mindatetime`, pid
    FROM child
    GROUP BY pid
) c
on p.id = c1.pid
JOIN child cmax
ON p.id = cmax.pid AND c.`maxdatetime` = cmax.`datetime`
JOIN child cmin
ON p.id = cmin.pid AND c.`mindatetime` = cmin.`datetime`