Retrieving the last record in each group not getting expected result - MySQL

Question

I want to fetch data of last two months of each category from table.

Table looks like:

Id Year month category value
1 2019 1 TEST1 10
2 2018 12 TEST1 10
3 2018 10 TEST1 10
4 2018 1 TEST2 10
5 2018 12 TEST2 10
6 2018 1 TEST3 10

Expected output:

Id Year month category value
1 2019 1 TEST1 10
2 2018 12 TEST1 10
5 2018 12 TEST2 10
4 2018 1 TEST2 10
6 2018 1 TEST3 10

I tried using:

SELECT a.year,a.month,a.value, a.category
FROM test_data AS a
WHERE 
(
SELECT COUNT(*)
FROM test_data AS b
WHERE b.category = a.category AND (b.year >= a.year AND b.month >= a.month)) <= 2
ORDER BY a.year DESC, a.month DESC

But it is giving extra record of TEST1 category. I guess because it is not working as expected for year condition. Please provide solution for this

Answer 1

Your first effort should go into fixing your data model and using a date-like datatype to store the date information, rather than spreading it over different columns. This should be as simple as adding a new column to the table and updating it from the existing columns:

alter table test_data add mydate date;
update test_data set mydate = concat(year, '-', month, '-01');

That said, to make your current query work you need to fine-tune the conditions on the dates as follows. One option uses arithmetics:

SELECT a.year, a.month, a.value, a.category
FROM test_data AS a
WHERE (
    SELECT COUNT(*)
    FROM test_data AS b
    WHERE 
        b.category = a.category 
        AND 100 * b.year + b.month >= 100 * a.year + a.month
) <= 2
ORDER BY a.year DESC, a.month DESC

Alternatively, if you are running MySQL 8.0, you can use row_number():

SELECT  a.year, a.month, a.value, a.category
FROM (
    SELECT 
        a.*,
        ROW_NUMBER() OVER(PARTITION BY a.category ORDER BY a.year DESC, a.month DESC)
    FROM test_data AS a 
) a
WHERE rn <= 2