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1.测试数据:
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| create table `t_class` (
`id` bigint (11),
`name` varchar (150),
`age` int (11),
`class` int (11)
);
insert into `t_class` (`id`, `name`, `age`, `class`) values('1','tom1','22','1');
insert into `t_class` (`id`, `name`, `age`, `class`) values('2','tom2','32','1');
insert into `t_class` (`id`, `name`, `age`, `class`) values('4','jerry2','33','2');
insert into `t_class` (`id`, `name`, `age`, `class`) values('5','hanchao','11','3');
insert into `t_class` (`id`, `name`, `age`, `class`) values('6','hanchao2','30','3');
insert into `t_class` (`id`, `name`, `age`, `class`) values('7','jerry3','32','2');
insert into `t_class` (`id`, `name`, `age`, `class`) values('8','jerry4','23','2');
insert into `t_class` (`id`, `name`, `age`, `class`) values('9','jerry1','33','2');
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2.需求:查找每个班年龄最大的学生的信息。
很明显,我们会用到group by,
eg:
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| SELECT id,NAME,MAX(age),class FROM t_class GROUP BY class;【错误的写法!!】
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但是,这是错误的写法!!
当然,我们也可以这么写:
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| SELECT * FROM (
SELECT * FROM t_class ORDER BY age DESC) AS b
GROUP BY class;
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但是,我不建议这么写,因为这么写有些不可预知的事情发生。我们先order by 再group by ,结果可能不是取的年龄最大的学生信息,具体原因,我还在寻找中,当然你知道原因也可以告诉我,证据如下:
所以,我们可以这么去写:
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| /**方法1:**/
SELECT * FROM t_class t WHERE t.age = (SELECT MAX(age) FROM t_class WHERE t.class = class) ORDER BY class;
/**方法2:**/
SELECT t_class.id,t_class.name,t_class.age,t_class.class FROM t_class
INNER JOIN (SELECT class,MAX(age) AS maxAge
FROM t_class GROUP BY class)t ON (t.class = t_class.class AND t_class.age = t.maxAge);
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至于那个方法的效率高,你可以自己测试一下!
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