MySQL 究极奥义·动态乾坤大挪移·无敌行列转换术

导入大SQL文件

[mysqld]
# 大批量导入优化
bulk_insert_buffer_size=1G
max_allowed_packet=1G
innodb_autoextend_increment=512M
innodb_buffer_pool_size=4G
innodb_log_buffer_size=4G
innodb_log_file_size=4G

动态行列转换

DROP TABLE IF EXISTS tb_score;CREATE TABLE tb_score(id INT(11) NOT NULL auto_increment,userid VARCHAR(20) NOT NULL COMMENT '用户id',subject VARCHAR(20) COMMENT '科目',score DOUBLE COMMENT '成绩',PRIMARY KEY(id)
)ENGINE = INNODB DEFAULT CHARSET = utf8;
INSERT INTO tb_score(userid,subject,score) VALUES ('001','语文',90);
INSERT INTO tb_score(userid,subject,score) VALUES ('001','数学',92);
INSERT INTO tb_score(userid,subject,score) VALUES ('001','英语',80);
INSERT INTO tb_score(userid,subject,score) VALUES ('002','语文',88);
INSERT INTO tb_score(userid,subject,score) VALUES ('002','数学',90);
INSERT INTO tb_score(userid,subject,score) VALUES ('002','英语',75.5);
INSERT INTO tb_score(userid,subject,score) VALUES ('003','语文',70);
INSERT INTO tb_score(userid,subject,score) VALUES ('003','数学',85);
INSERT INTO tb_score(userid,subject,score) VALUES ('003','英语',90);
INSERT INTO tb_score(userid,subject,score) VALUES ('003','政治',82);

select * from test.tb_score;selectuserid,sum(if(subject='语文', score, 0)) as '语文',sum(if(subject='数学', score, 0)) as '数学',sum(if(subject='英语', score, 0)) as '英语'
from test.tb_score
group by userid;SET @EE='';
select @EE :=CONCAT(@EE,'sum(if(subject= \'',subject,'\',score,0)) as ',subject, ',') AS aa FROM (SELECT DISTINCT subject FROM tb_score) A ;SET @QQ = CONCAT('select ifnull(userid,\'TOTAL\')as userid,',@EE,' sum(score) as TOTAL from tb_score group by userid WITH ROLLUP');
-- SELECT @QQ;PREPARE stmt FROM @QQ;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

递归
开发中经常会遇到树形结构，例如菜单、分类等，我们通常需要在代码中使用递归的方式组织树形数据。
现在有一张课程分类表，需要查询给定分类的所有子分类。
课程表如下：

create table course_category
(id       varchar(20)             not null comment '主键'primary key,name     varchar(32)             not null comment '分类名称',label    varchar(32)             null comment '分类标签默认和名称一样',parentid varchar(20) default '0' not null comment '父结点id（第一级的父节点是0，自关联字段id）',is_show  tinyint                 null comment '是否显示',orderby  int                     null comment '排序字段'
)comment '课程分类' charset = utf8;

现在系统中最多只有三级分类，可以直接进行两次表关联实现。

select a.name, b.name, c.name
from course_category ajoin course_category b on a.id = b.parentidjoin course_category c on b.id = c.parentid
where a.id = '1';

使用Mysql递归语法。
上面的方法显然不够通用，随着分类层级的变化SQL也需要更改，在Mysql 8.0中提供了递归语法，轻松实现树形结构的查询。

with recursive t1 as (-- 递归的上下文变量select 1 as nunion all-- 递归的查询，每次递归都会基于该sql的结果继续，直到where条件不满足停止递归。select n + 1 from t1 where n < 5
)
select * from t1;

执行上面的sql将会得到：

n
1
2
3
4
5

想要查询id='1’的分类（根节点）下所有的子分类的SQL如下：

with recursive t1 as (select * from course_category where id = '1'union allselect a.* from course_category a join t1 on t1.id = a.parentid
)
select * from t1;