SQL:在多列上使用GROUP BY和MAX [英] SQL : Using GROUP BY and MAX on multiple columns

问题描述

我在使用SQL查询时遇到问题.让我们以这个示例数据为例

I have an issue with an SQL Query. Lets take this example data

itemID  catID  attrib1  attrib2
  1       1       10       5   
  2       1       10       7
  3       1        5      10
  4       2       18      15

我想返回每个类别的最佳商品(attrib1优先于attrib2)

I want to return the best item for each category (with attrib1 having priority over attrib2)

很显然，SELECT catID, MAX(attrib1), MAX(attrib2) FROM test_table GROUP BY catID不起作用，因为它将返回10& ;;.第一只猫10.

Obviously, SELECT catID, MAX(attrib1), MAX(attrib2) FROM test_table GROUP BY catID doesn't work since it will return 10 & 10 for the 1st cat.

所以有什么办法告诉MySQL从attrib2行中选择最大值，而仅考虑attrib1也是最大值的情况吗?即返回以下数据

So is there anyway to tell MySQL to select max value from attrib2 row but only consider the ones where attrib1 is also max value ? i.e return the following data

 catID  attrib1  attrib2
   1       10       7   
   2       18      15

推荐答案

您可以获取最佳的attrib1值，然后加入attrib2值并为每个attrib1值获取最佳值:

You can get the best attrib1 values, and then join in the attrib2 values and get the best of those for each attrib1 value:

select t2.catID, t2.attrib1, max(t2.attrib2)
from
(
  select catID, max(attrib1) as attrib1
  from test_table
  group by catID
) t1
inner join test_table t2 on t2.catID = t1.catID and t2.attrib1 = t1.attrib1
group by t2.catID, t2.attrib1

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