sql 查询出各科成绩、平均成绩，总成绩，根据总成绩分数相同，排名相同

今天面试被sql绊倒了，废话少说，希望以后面试时不再填坑。需求基本和标题一样。

三张表：学生表student，科目表：course，成绩表：grade

sql语句如下

CREATE TABLE `course` (
  `course_id` int(11) NOT NULL AUTO_INCREMENT,
  `c_name` varchar(64) NOT NULL,
  PRIMARY KEY (`course_id`)
)
INSERT INTO `course` VALUES ('1', '语文');
INSERT INTO `course` VALUES ('2', '数学');
INSERT INTO `course` VALUES ('3', '外语');


CREATE TABLE `grade` (
  `grade_id` int(11) NOT NULL AUTO_INCREMENT,
  `student_id` int(11) NOT NULL,
  `course_id` int(11) NOT NULL,
  `score` decimal(5,2) NOT NULL,
  PRIMARY KEY (`grade_id`)
)

INSERT INTO `grade` VALUES ('1', '1', '1', '83.00');
INSERT INTO `grade` VALUES ('2', '1', '2', '75.00');
INSERT INTO `grade` VALUES ('3', '1', '3', '59.00');
INSERT INTO `grade` VALUES ('4', '2', '1', '76.00');
INSERT INTO `grade` VALUES ('5', '2', '2', '95.00');
INSERT INTO `grade` VALUES ('6', '2', '3', '87.00');
INSERT INTO `grade` VALUES ('7', '3', '1', '89.00');
INSERT INTO `grade` VALUES ('8', '3', '2', '74.00');
INSERT INTO `grade` VALUES ('9', '3', '3', '58.00');
INSERT INTO `grade` VALUES ('10', '4', '1', '95.00');
INSERT INTO `grade` VALUES ('11', '4', '2', '76.00');
INSERT INTO `grade` VALUES ('12', '4', '3', '87.00');


CREATE TABLE `student` (
  `student_id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(16) NOT NULL,
  `age` int(11) DEFAULT NULL,
  PRIMARY KEY (`student_id`)
)

INSERT INTO `student` VALUES ('1', '张三', '18');
INSERT INTO `student` VALUES ('2', '李四', '18');
INSERT INTO `student` VALUES ('3', '王五', '18');
INSERT INTO `student` VALUES ('4', '赵柳', '18');

首先行变列查询出其中的各科成绩以及平均成绩、总成绩：

select s.student_id as '编号',s.name as '姓名',s.age as '年龄',
sum(case c_name when '语文' then score else 0 end) as '语文',
sum(case c_name when '数学' then score else 0 end) as '数学',
sum(case c_name when '外语' then score else 0 end) as '外语',
convert(sum(score)/3,decimal(5,2)) as '平均成绩',
convert(sum(score),decimal(5,2)) as total
from student s,course c,grade g where s.student_id=g.student_id and c.course_id=g.course_id
group by s.student_id order by sum(score) desc

sql语法慢慢体会，还是不难的，主要因为之前很久没看过了，陌生了。。。

然后需要查出排名，而且总分相同的排名相同，好的，此时运用到了sql变量以及赋值

sql变量用@来表示，赋值用:=来实现；

select
@rows:=@rows+1 as rows,
if(@gnum=total,@rownum:=@rownum,@rownum:=@rownum+1) as rank,
@gnum:=total,
 message.* from(
select s.student_id as '编号',s.name as '姓名',s.age as '年龄',
sum(case c_name when '语文' then score else 0 end) as '语文',
sum(case c_name when '数学' then score else 0 end) as '数学',
sum(case c_name when '外语' then score else 0 end) as '外语',
convert(sum(score)/3,decimal(5,2)) as '平均成绩',
convert(sum(score),decimal(5,2)) as total
from student s,course c,grade g where s.student_id=g.student_id and c.course_id=g.course_id
group by s.student_id order by sum(score) desc) message,(select @rownum:=0,@gnum:=0,@rows:=0) number

解析一下首先开始时候 @gnum表示上个总成绩变量初始化=0，然后判断是否和上个total相同相同的话，把上个排名@rownnum赋值给当前列排名，不同则@rownum+1

查询结果如下:

基础还要掌握。。

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