Mysql进阶

如图一,通过sq了实现图二:

图一如下:

图二如下:

附建表语句&初始化数据语句：

CREATE TABLE `sc` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `user_name` varchar(32) NOT NULL DEFAULT '',
  `subject` varchar(32) NOT NULL DEFAULT '',
  `score` int(11) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

insert into sc(id,user_name,subject,score) 
values (1,"张三","语文",80),(2,"张三","数学",90),(3,"张三","英语",70),(4,"张三","生物",85),
       (5,"李四","语文",80),(6,"李四","数学",90),(7,"李四","英语",70),(8,"李四","生物",85);

以下有简单易懂的&万能通用的两种写法:

 1.简单易懂的一种写法，由于一名学生一门课只有一个分数，所以下面sql中max函数完全可以换为sum等其他聚合函数。

select `user_name`,
    max(case `subject` when '语文' then IFNULL(`score`, 0) else 0 end) as '语文',
    max(case `subject` when '数学' then IFNULL(`score`, 0) else 0 end) as '数学',
    max(case `subject` when '英语' then IFNULL(`score`, 0) else 0 end) as '英语',
    max(case `subject` when '生物' then IFNULL(`score`, 0) else 0 end) as '生物'
from `sc`
group by `user_name`;

运行结果:

2.不需要提前知道有多少门课程，动态生成列，万能通用的一种写法。

set @splice_sql = null;
SELECT 
    GROUP_CONCAT(DISTINCT
        CONCAT('max(if(subject=''',subject,''', score, 0)) as ''',subject, ''''))
    into @splice_sql
from sc;

set @splice_sql = CONCAT('select user_name,', @splice_sql, ' from sc group by user_name');
prepare bella_test from @splice_sql;
execute bella_test;
DEALLOCATE prepare bella_test;

运行结果