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1.场景

需要写一个sql,需要合并 从四张表查询查询出来的结果，需要用到 UNION ALL 对查询结果进行合并

2.sql

-- 从 unit 表中查询
select id, name, '1' as type
from unit
where name like concat('%', #{name}, '%')UNION ALL-- 从 system 表中查询
select id, name, '2' as type
from system
where name like concat('%', #{name}, '%')UNION ALL-- 从 equipment 表中查询
select id, name, '3' as type
from equipment
where name like concat('%', #{name}, '%')UNION ALL-- 从 measuring_point 表中查询
select id, name, '4' as type
from measuring_point
where name like concat('%', #{name}, '%')

3.查询报错

java.sql.SQLException: Illegal mix of collations for operation 'UNION'

4.检查4张建表语句的排序规则是否一致

SHOW FULL COLUMNS FROM unit;
SHOW FULL COLUMNS FROM system;
SHOW FULL COLUMNS FROM equipment;
SHOW FULL COLUMNS FROM measuring_point;

CREATE TABLE `template_config_unit` (`id` bigint NOT NULL AUTO_INCREMENT COMMENT '主键',`template_name` varchar(255) COLLATE utf8mb4_general_ci NOT NULL COMMENT '机组模板名称',`code` varchar(100) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL COMMENT '编码',`type` int DEFAULT NULL COMMENT '机组类型（1:锅炉专业, 2:汽机专业, 3:电气专业, 4:环化专业）'PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=227 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci COMMENT='机组表';

重点关注

ENGINE=InnoDB AUTO_INCREMENT=227 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci 

5.本人遇到的问题 是排序规则不一致造成的

6.解决方案

修改表的排序规则

ALTER TABLE unit CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;
ALTER TABLE system CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;
ALTER TABLE equipment CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;
ALTER TABLE measuring_point CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;

7.最后

问题解决