SQL:在多列上使用GROUP BY和MAX

本文介绍了SQL:在多列上使用GROUP BY和MAX的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 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