一文弄懂MYSQL如何列转行

一、需求：

有三张表，学生表、成绩表和课程表，我们可以通过连表查询出学生姓名、课程及对应的成绩：所需表sql

-- ----------------------------
-- table structure for student
-- ----------------------------
drop table if exists `student`;
create table `student` (
  `s_id` varchar(20) not null default '',
  `s_name` varchar(20) not null default '',
  `s_birth` varchar(20) not null default '',
  `s_sex` varchar(10) not null default '',
  primary key (`s_id`)
) engine=innodb default charset=utf8;
 
-- ----------------------------
-- records of student
-- ----------------------------
insert into `student` values ('01', '赵雷', '1990-01-01', '男');
insert into `student` values ('02', '钱电', '1990-12-21', '男');
insert into `student` values ('03', '孙风', '1990-05-20', '男');
insert into `student` values ('04', '李云', '1990-08-06', '男');
insert into `student` values ('05', '周梅', '1991-12-01', '女');
insert into `student` values ('06', '吴兰', '1992-03-01', '女');
insert into `student` values ('07', '郑竹', '1989-07-01', '女');
insert into `student` values ('08', '王菊', '1990-01-20', '女');
 
-- ----------------------------
-- table structure for course
-- ----------------------------
drop table if exists `course`;
create table `course` (
  `c_id` varchar(20) not null default '',
  `c_name` varchar(20) not null default '',
  `t_id` varchar(20) not null,
  primary key (`c_id`)
) engine=innodb default charset=utf8;
 
-- ----------------------------
-- records of course
-- ----------------------------
insert into `course` values ('01', '语文', '02');
insert into `course` values ('02', '数学', '01');
insert into `course` values ('03', '英语', '03');
 
-- ----------------------------
-- table structure for score
-- ----------------------------
drop table if exists `score`;
create table `score` (
  `s_id` varchar(20) not null default '',
  `c_id` varchar(20) not null default '',
  `s_score` int(3) default null,
  primary key (`s_id`,`c_id`)
) engine=innodb default charset=utf8;
 
-- ----------------------------
-- records of score
-- ----------------------------
insert into `score` values ('01', '01', '80');
insert into `score` values ('01', '02', '90');
insert into `score` values ('01', '03', '99');
insert into `score` values ('02', '01', '70');
insert into `score` values ('02', '02', '60');
insert into `score` values ('02', '03', '80');
insert into `score` values ('03', '01', '80');
insert into `score` values ('03', '02', '80');
insert into `score` values ('03', '03', '80');
insert into `score` values ('04', '01', '50');
insert into `score` values ('04', '02', '30');
insert into `score` values ('04', '03', '20');
insert into `score` values ('05', '01', '76');
insert into `score` values ('05', '02', '87');
insert into `score` values ('06', '01', '31');
insert into `score` values ('06', '03', '34');
insert into `score` values ('07', '02', '89');
insert into `score` values ('07', '03', '98');

select s.s_id,s.s_name,c.c_name,sc.s_score 
from student s 
left join score sc on sc.s_id = s.s_id 
left join course c on c.c_id = sc.c_id

好的，现在呢我们要把课程名称呢变成横行呢？

二、如何实现

1）首先看我们的静态sql

关联成绩表课程表查询学生各科课程成绩

select s.s_id,s.s_name,c.c_name,sc.s_score
from student s
left join score sc on sc.s_id=s.s_id
left join course c on c.c_id = sc.c_id;

if(s1,s2,s3)表达式,类似三木运算符取值,s1值为真取s2值,假取s3个值,最后可得到某一科成绩

select  p.s_id,p.s_name, p.c_name,p.c_name = '数学',
if(p.c_name = '数学',p.c_name,null)c_name,if(p.c_name = '数学',p.s_score,null)s_score
from (
	select s.s_id,s.s_name,c.c_name,sc.s_score
	from student s
	left join score sc on sc.s_id=s.s_id
	left join course c on c.c_id = sc.c_id	)p;

然后我们分组且用max函数获取每个学生的数学课程的成绩,替换这一课的字段名称

select  p.s_id,
        p.s_name, 
        max(if(p.c_name = '数学', p.s_score, null)) as 数学
from (
	select s.s_id,s.s_name,c.c_name,sc.s_score
	from student s
	left join score sc on sc.s_id=s.s_id
	left join course c on c.c_id = sc.c_id	)p
group by p.s_id;

获取所有人各科成绩

select  p.s_id,
        p.s_name, 
        max(if(p.c_name = '数学', p.s_score, null)) as 数学,
        max(if(p.c_name = '语文', p.s_score, null)) as 语文,
        max(if(p.c_name = '英语', p.s_score, null)) as 英语
from (
	select s.s_id,s.s_name,c.c_name,sc.s_score
	from student s
	left join score sc on sc.s_id=s.s_id
	left join course c on c.c_id = sc.c_id	)p
group by p.s_id;

2)那么就有人问了，如果我有100门课程不是要写100次名称，这也太麻烦了？

接下来请看动态sql

我们的动态sql是拼接实现的, 主要就是拼接我们的课程成绩那一句, 所以要先看一下concat函数拼接课程语句

select c_name,concat( 'max(if(p.c_name = ''', c_name, ''', c.s_score, null)) as ', c_name ) from course c;

是的，结果就是上面要的max函数

然后我么可以用group_concat()函数把这些内容拼接成一句

select group_concat(distinct c_name,concat( 'max(if(p.c_name = ''', c_name, ''', c.s_score, null)) as ', c_name )) from course c;

接下来，拼接sql实现需求

-- 1.定义一个sql变量
set @sql = null;
 
-- 2.把我们的查询课程的sql赋给变量
select group_concat(distinct concat('max(if(p.c_name = ''',c_name,''', p.s_score, null)) as ',c_name)) into @sql
from course;
 
-- 3.拼接sql
set @sql = concat('select  p.s_id, p.s_name, ', @sql ,'
			from (select s.s_id,s.s_name,c.c_name,sc.s_score 
			from student s 
			left join score sc on sc.s_id=s.s_id 
			left join course c on c.c_id = sc.c_id)p 
			group by p.s_id');
 
-- 预处理语句 
prepare stmt from @sql;
-- 执行
execute stmt;
-- 销毁
deallocate prepare stmt;

3)这样每次都写一长串sql也很麻烦？

好的那么我们来封装成存储过程

-- 1、创建无参存储过程
delimiter $$
create procedure getstudentrow()
begin
    ------把要执行的sql放在这里就可以了
		set @sql = null;
		select
        group_concat(distinct concat('max(if(p.c_name = ''',c_name,''', p.s_score, null)) as ',c_name)) 
        into @sql from course;
    set @sql = concat('select  p.s_id, p.s_name, ', @sql ,'
    	from (select s.s_id,s.s_name,c.c_name,sc.s_score 
    	from student s 
    	left join score sc on sc.s_id=s.s_id 
    	left join course c on c.c_id = sc.c_id)p 
    	group by p.s_id');
    prepare stmt from @sql;
    execute stmt;
    deallocate prepare stmt;
    ------把要执行的sql放在这里就可以了
end$$;
delimiter;
 
-- 查询存储过程
show procedure status;
 
-- 调用
call getstudentrow();

这样每次直接调用就可以了？

总结

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