我如何获得每个外键的最小值

Question

Given the following table:

student   discipline   mark
-------   ----------   ----
   1         math       5 
   1      phylosophy    4
   1      literature    3
   2         math       2
   2      phylosophy    5
   2      literature    5

What is the best way to get the minimal mark for each student? (result should be [3,2])

Answer 1

Use the MIN function.

SELECT student, MIN(mark)
FROM result_table
GROUP BY student

If you need the discipline they got the lowest mark in you can do the following:

SELECT result_table.*
FROM result_table 
JOIN (SELECT student, MIN(mark) as min_mark
  FROM result_table
  GROUP BY student) lowest_result ON result_table.student = lowest_result.student 
AND result_table.mark = lowest_result.min_mark

This will show the results where the student had the lowest mark. Note that this will return two rows for a student if they have the same lowest mark in multiple subject. To avoid this you can add another MIN around the discipline and GROUP BY student and mark.

Answer 2

If you need the discipline as part of the output the following might be slightly faster than using a sub-select (because only a single scan over the table is necessary) but it will probably only show for larger tables.

select student,
       discipline, 
       mark as lowest_mark
from (
   select student,
          discipline,
          mark,
          row_number() over (partition by student order by mark) as rn
   from the_table
) t
where rn = 1

It will always return exactly one row per student. If there are two disciplines with the same mark, it's not defined which one will be taken.

If you do want to return multiple rows if the lowest mark occurs more than once, you can use this:

select student,
       discipline, 
       mark as lowest_mark
from (
   select student,
          discipline,
          mark,
          min(mark) over (partition by student) as min_mark
   from the_table
) t
where mark = min_mark

If you do not need the discipline, but only the lowest mark, then GavinCattell's first statement is the way to go.