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法律研究在 MySQL 中对多列使用 GROUP BY 和 MAX？
为了理解针对多列的 GROUP BY 和 MAX，我们先创建一个表。创建表的查询如下 −
mysql> create table GroupByMaxDemo -> ( -> Id int NOT NULL AUTO_INCREMENT PRIMARY KEY, -> CategoryId int, -> Value1 int, -> Value2 int -> ); Query OK, 0 rows affected (0.68 sec)
示例
使用 insert 命令插入一些记录到表中。查询如下 −
mysql> insert into GroupByMaxDemo(CategoryId, Value1,Value2) values(10,100,50); Query OK, 1 row affected (0.15 sec) mysql> insert into GroupByMaxDemo(CategoryId, Value1,Value2) values(10,100,70); Query OK, 1 row affected (0.21 sec) mysql> insert into GroupByMaxDemo(CategoryId, Value1,Value2) values(10,50,100); Query OK, 1 row affected (0.22 sec) mysql> insert into GroupByMaxDemo(CategoryId, Value1,Value2) values(20,180,150); Query OK, 1 row affected (0.19 sec)
使用 select 语句从表中显示所有记录。查询如下 −
mysql> select *from GroupByMaxDemo;
输出
+----+------------+--------+--------+ | Id | CategoryId | Value1 | Value2 | +----+------------+--------+--------+ | 1 | 10 | 100 | 50 | | 2 | 10 | 100 | 70 | | 3 | 10 | 50 | 100 | | 4 | 20 | 180 | 150 | +----+------------+--------+--------+ 4 rows in set (0.00 sec)
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示例
以下是对多列使用 GROUP BY 和 MAX 的查询 −
mysql> select tbl2.CategoryId, tbl2.Value1, max(tbl2.Value2) -> from -> ( -> select CategoryId, max(Value1) as `Value1` -> from GroupByMaxDemo -> group by CategoryId -> ) tbl1 -> inner join GroupByMaxDemo tbl2 on tbl2.CategoryId = tbl1.CategoryId and tbl2.Value1 = tbl1.Value1 -> group by tbl2.CategoryId, tbl2.Value1;
输出
+------------+--------+------------------+ | CategoryId | Value1 | max(tbl2.Value2) | +------------+--------+------------------+ | 10 | 100 | 70 | | 20 | 180 | 150 | +------------+--------+------------------+ 2 rows in set (0.00 sec)
更新时间： 26-6 月 2020 年
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