遇到一个场景，要把数据分组，然后获取每组前10条数据，首先我想到用group by分组，但是难点是分组后怎么知道该数据在组里面排第几条。
一、创建表，插入相关测试数据
CREATE TABLE `score` ( `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '主键', `subject` varchar(20) DEFAULT NULL COMMENT '科目', `student_id` int(11) DEFAULT NULL COMMENT '学生id', `student_name` varchar(20) NOT NULL COMMENT '学生姓名', `score` double DEFAULT NULL COMMENT '成绩', PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=16 DEFAULT CHARSET=utf8;
备注：插入的数据sql在最后面，小伙伴可以自行验证下面的sql
二、查询每科成绩前三的记录
数据有了，那么写sql,sql如下：
###每科成绩前三名 SELECT * FROM score s1 WHERE ( SELECT count( * ) FROM score s2 WHERE s1.`subject` = s2.`subject` AND s1.score < s2.score ) < 3 ORDER BY SUBJECT, score DESC
分析：
里面用了子查询，核心sql是where后面的这个条件：
( SELECT count( * ) FROM score s2 WHERE s1.subject = s2.subject AND s1.score < s2.score ) < 3
这段sql的意思是。。。
感觉我的语言有点描述不出来，还是用我熟悉的java代码描述上面的sql,大概就是for循环遍历两次，在第二次for循环的时候统计同一科目的学生记录，比s1的学生分数高的数量，如果这个数量小于3的话，说明s1排名前三，看下面的代码理解理解
public class StudentTest {
public static void main(String[] args) {
List list = new ArrayList<>();
//初始化和表结构一致的数据
initData(list);
//记录查询出来的结果
List result = new ArrayList<>();
for(Student s1 : list){
int num = 0;
//两次for循环遍历，相当于sql里面的子查询
for(Student s2:list){
//统计同一科目，且分数s2分数大于s1的数量，简单理解就是同一科目的学生记录，比s1的学生分数高的数量
if(s1.getSubject().equals(s2.getSubject())
&&s1.getScore() System.out.println(String.format("学生:%s,科目：%s,成绩:%s",s.getName(),s.getSubject(),s.getScore()))
);
}
public static void initData(List list) {
list.add(new Student(1,"语文","张三",59));
list.add(new Student(2,"数学","张三",78));
list.add(new Student(3,"英语","张三",65));
list.add(new Student(4,"语文","李四",88));
list.add(new Student(5,"数学","李四",58));
list.add(new Student(6,"英语","李四",65));
list.add(new Student(7,"语文","王五",92));
list.add(new Student(8,"数学","王五",99));
list.add(new Student(9,"英语","王五",96));
list.add(new Student(10,"语文","小张",90));
list.add(new Student(11,"数学","小张",91));
list.add(new Student(12,"英语","小张",90));
list.add(new Student(13,"语文","小华",88));
list.add(new Student(14,"数学","小华",79));
list.add(new Student(15,"英语","小华",77));
}
@Data
public static class Student {
private int id;
private String subject;
private String name;
private double score;
//想当于表结构
public Student(int id, String subject, String name, double score) {
this.id = id;
this.subject = subject;
this.name = name;
this.score = score;
}
} 可以看到代码运行完打印出来的结果和执行sql后的结果是一样的
三、查询学生各科分数大于等于90分的记录
表和数据都有了，顺便也总结一些这类型的sql题
如题目为查询上面表的各科成绩都大于等于90分的记录，那么sql怎么写？
1. 第一种写法：正向思考
各科成绩都大于90分的，那么最低分的也必须大于等于90分，sql如下：
SELECT * FROM score WHERE student_id IN (SELECT student_id FROM score GROUP BY student_id HAVING min( score ) >= 90 )
2. 第二种写法：逆向思考
排除最高分都小于90分的记录
SELECT * FROM score WHERE student_id NOT IN (SELECT student_id FROM score GROUP BY student_id HAVING max( score ) < 90 )
备注：正向和逆向看具体情况选择
其他的插叙
查询学生各科平均分大于80分的记录
###查询学生各科平均分大于80分的记录
select * from score where student_id in(
select student_id from score GROUP BY student_id HAVING avg(score)>80
)查询一个学生每科分数不及格的记录
###查询一个学生每科分数不及格的记录 SELECT * FROM score WHERE student_id IN ( SELECT student_id FROM score GROUP BY student_id HAVING max( score ) < 60 )
附：表结构插入的sql
CREATE TABLE `score` ( `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '主键', `subject` varchar(20) DEFAULT NULL COMMENT '科目', `student_id` int(11) DEFAULT NULL COMMENT '学生id', `student_name` varchar(20) NOT NULL COMMENT '学生姓名', `score` double DEFAULT NULL COMMENT '成绩', PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=16 DEFAULT CHARSET=utf8; INSERT INTO `test`.`score`(`id`, `subject`, `student_id`, `student_name`, `score`) VALUES (1, '语文', 1, '张三', 59); INSERT INTO `test`.`score`(`id`, `subject`, `student_id`, `student_name`, `score`) VALUES (2, '数学', 1, '张三', 78); INSERT INTO `test`.`score`(`id`, `subject`, `student_id`, `student_name`, `score`) VALUES (3, '英语', 1, '张三', 65); INSERT INTO `test`.`score`(`id`, `subject`, `student_id`, `student_name`, `score`) VALUES (4, '语文', 2, '李四', 88); INSERT INTO `test`.`score`(`id`, `subject`, `student_id`, `student_name`, `score`) VALUES (5, '数学', 2, '李四', 58); INSERT INTO `test`.`score`(`id`, `subject`, `student_id`, `student_name`, `score`) VALUES (6, '英语', 2, '李四', 65); INSERT INTO `test`.`score`(`id`, `subject`, `student_id`, `student_name`, `score`) VALUES (7, '语文', 3, '王五', 92); INSERT INTO `test`.`score`(`id`, `subject`, `student_id`, `student_name`, `score`) VALUES (8, '数学', 3, '王五', 99); INSERT INTO `test`.`score`(`id`, `subject`, `student_id`, `student_name`, `score`) VALUES (9, '英语', 3, '王五', 96); INSERT INTO `test`.`score`(`id`, `subject`, `student_id`, `student_name`, `score`) VALUES (10, '语文', 4, '小张', 90); INSERT INTO `test`.`score`(`id`, `subject`, `student_id`, `student_name`, `score`) VALUES (11, '数学', 4, '小张', 91); INSERT INTO `test`.`score`(`id`, `subject`, `student_id`, `student_name`, `score`) VALUES (12, '英语', 4, '小张', 90); INSERT INTO `test`.`score`(`id`, `subject`, `student_id`, `student_name`, `score`) VALUES (13, '语文', 5, '小华', 88); INSERT INTO `test`.`score`(`id`, `subject`, `student_id`, `student_name`, `score`) VALUES (14, '数学', 5, '小华', 79); INSERT INTO `test`.`score`(`id`, `subject`, `student_id`, `student_name`, `score`) VALUES (15, '英语', 5, '小华', 77);
