How to get MAX date from table join that use UNION? - mysql

Question

I have 3 table that need to be joined to get the max date.
table_grade_A
ID_GRADE GRADE NOTE SURVEYOR
1 70.7 PASS TOM
3 51.2 FAIL TOM
table_grade_B
ID_GRADE SUB_GRADE_I SUB_GRADE_II TOTAL_GRADE NOTE SURVEYOR
2 30.8 40.1 70.9 PASS MARVOLO
4 10.3 54.1 64.4 FAIL MARVOLO
5 41.7 20.9 62.6 FAIL RIDDLE
table_grade
ID_GRADE STUDENT TEST_DATE
1 MIYA 2018-12-20
2 LAYLA 2018-12-21
3 MIYA 2018-12-21
4 MIYA 2018-12-22
5 KARRIE 2018-12-28
Every student may get different test and different test stored in different table. I use UNION to populate the value from table_grade_a and table_grade_b and JOIN them to table_grade
My current query:
SELECT tg.STUDENT, MAX(tg.TEST_DATE) AS 'TEST_DATE', temp_grade.* FROM `table_grade` tg
INNER JOIN (
SELECT ID_GRADE,GRADE,NOTE
FROM table_grade_a 'tga'
UNION ALL
SELECT ID_GRADE,TOTAL_GRADE AS GRADE,NOTE
FROM table_grade_a 'tgb'
)temp_grade ON tg.ID_GRADE = temp_grade.ID_GRADE
WHERE tg.STUDENT = 'MIYA'
The result of above query is:
STUDENT TEST_DATE GRADE NOTE
MIYA 2018-12-22 70.7 PASS
The expected output should be:
STUDENT TEST_DATE GRADE NOTE
MIYA 2018-12-22 64.4 FAIL

Answer 1

For a result corresponding the the max date of each student:
The MIN or MAX of a column does not necessarily align to the other values of the wanted row(s), so you need to do more than just calculate the maximum date. In MySQL prior to version 8 you could do something like this, by calculating the maximum dates then using that as an inner join to limit the rows to those corresponding to the maximum values:
select
temp_grade .*
from table_grade tg
inner join (
select student, max(test_date) as test_date
from table_grade
group by student
) gd on tg.student = gd.student and tg.test_date = gd.test_date
INNER JOIN (
SELECT ID_GRADE,GRADE,NOTE
FROM table_grade_a 'tga'
UNION ALL
SELECT ID_GRADE,TOTAL_GRADE AS GRADE,NOTE
FROM table_grade_a 'tgb'
)temp_grade ON tg.ID_GRADE = temp_grade.ID_GRADE
# WHERE tg.STUDENT = 'MIYA'
In MySQL v8+ you could use row_number() over(...) instead:
select
temp_grade .*
from (
select *
, row_number() over(partition by student order by test_date DESC) as rn
from table_grade
) tg
INNER JOIN (
SELECT ID_GRADE,GRADE,NOTE
FROM table_grade_a 'tga'
UNION ALL
SELECT ID_GRADE,TOTAL_GRADE AS GRADE,NOTE
FROM table_grade_a 'tgb'
)temp_grade ON tg.ID_GRADE = temp_grade.ID_GRADE
where tg.rn = 1
# and tg.STUDENT = 'MIYA'

Answer 2

The problem with your current approach is that you are selecting the max date, a table level aggregate, while also asking for all individual records at the same time. This does make sense. One correct possibility would be to use LIMIT with ORDER BY:
SELECT tg1.STUDENT, tg1.TEST_DATE, tg2.*
FROM table_grade tg1
INNER JOIN
(
SELECT ID_GRADE, GRADE, NOTE
FROM table_grade_a
UNION ALL
SELECT ID_GRADE, TOTAL_GRADE, NOTE
FROM table_grade_b
) tg2
ON tg1.ID_GRADE = tg2.ID_GRADE
WHERE
tg1.STUDENT = 'MIYA'
ORDER BY
tg1.TEST_DATE DESC
LIMIT 1;