Question

I have a MySQL table structured somewhat like this:

type    name    value
=====================
1       item1    1
1       item2    20
2       item3    0
3       item4    5
3       item5    2
3       item6    50

I need to write a query that returns the lowest valued item of each type, sorted by the value. So the result should be:

type    name    value
=====================
2       item3    0
1       item1    1
3       item5    2

I can get this to work, but it’s looking really, really ugly right now. Is there an elegant way to do this?

Thanks so much!

Answer 1

This is called a “greatest-n-per-group” query (under that tag on SO you will find many similar questions). (I know you want the “lowest-n-per-group”, but it’s the same problem).

Usually, you would be able to do:

SELECT type, MIN(value)
FROM mytable
GROUP BY type

But this won’t work if you also want the name corresponding to the MIN(value).

To retrieve the minimum value per type and also the corresponding row, you join your table to itself within type (the GROUP BY) variable, and with a sorting condition on value (the MIN) variable:

SELECT t1.type, t1.name, t1.value
FROM mytable t1
LEFT JOIN mytable t2 ON t1.type = t2.type AND t1.value > t2.value
WHERE t2.value IS NULL

Note :

• we LEFT JOIN mytable to itself, restricting the join such that all the types are the same. This will produce a table with every combination of values for each type.
• we add a condition to the LEFT JOIN to restrict the combinations of values such that t1.value > t2.value. So now we have a table with every combination of values within each type, but t1’s is bigger than t2’s
• since this is a LEFT JOIN, if there is a t1.value for which there is no smaller t2.value, the corresponding t2 columns will be NULL. But this is precisely the smallest t1.value for that type!
• Add a WHERE t2.value IS NULL condition in to pick out exactly these rows.