MySQL 联表查询去重 & 解决 DISTINCT 异常：Expression #1 of ORDER BY clause is not in SELECT list, references...

    在 MySQL 数据库中进行多表联合查询时，时常会碰到有重复值的情况，需要去重操作。下面介绍几种去重方法。

一、MySQL 联表查询去重

1. 使用distinct关键字

SELECT DISTINCT *
FROM table1
JOIN table2
ON table1.id = table2.id;

2. 使用GROUP BY

SELECT *
FROM table1
JOIN table2
ON table1.id = table2.id
GROUP BY table1.id;

3. 使用子查询

SELECT *
FROM (SELECT *
FROM table1
GROUP BY id) sub1
JOIN (SELECT *
FROM table2
GROUP BY id) sub2
ON sub1.id = sub2.id;

二、解决 DISTINCT 与 ORDER BY 联用抛出异常

    DISTINCT 与 ORDER 联合使用时，用于排序的字段（order by sort_num）必须要在 select list 里，否则会抛出类似下面的 MySQL 异常提示：

Caused by: java.sql.SQLException: Expression #1 of ORDER BY clause is not in SELECT list, references column 'xxx.x.sort_num' which is not in SELECT list; this is incompatible with DISTINCT

解决步骤：

1.  查看是否开启了 ONLY_FULL_GROUP_BY 规则校验

SELECT @@GLOBAL.sql_mode;
// 查询结果:
ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION

SELECT @@SESSION.sql_mode;
// 查询结果:
ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION

2. 关闭 ONLY_FULL_GROUP_BY 规则校验

// 去掉“ONLY_FULL_GROUP_BY,”后的值
set @@GLOBAL.sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';

set @@SESSION.sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';

3. 重启 MySQL 服务器

# 该令如执行失败，会提示：Redirecting to /bin/systemctl start mysql.service 
service mysqld start;

# 上面命令如果执行失败，可改用此命令
systemctl start mysqld