CREATE TABLE `user_info` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(255) DEFAULT NULL, `age` int(11) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 COMMENT='用户表'; INSERT INTO `test`.`user_info`(`id`, `name`, `age`) VALUES (1, 'aaa', 11); INSERT INTO `test`.`user_info`(`id`, `name`, `age`) VALUES (2, 'bbb', 22); INSERT INTO `test`.`user_info`(`id`, `name`, `age`) VALUES (3, 'ccc', 33);
数据以下
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union操做mysql
select id FROM user_info WHERE age = 11 UNION SELECT id FROM user_info WHERE age = 22 结果 +----+ | id | +----+ | 1 | | 2 | +----+
union allsql
mysql> select id FROM user_info WHERE age > 11 UNION ALL SELECT id FROM user_info WHERE age > 22; +----+ | id | +----+ | 2 | | 3 | | 3 | +----+
结论:union会去除重复,union all不会去除重复的数据;code
筛选的列数不一致htm
mysql> select id,name FROM user_info WHERE age = 11 UNION ALL SELECT id FROM user_info WHERE age = 22; 报错 1222 - The used SELECT statements have a different number of columns
筛选列数起别名blog
mysql> select id,name name1 FROM user_info WHERE age = 11 UNION ALL SELECT id,name name2 FROM user_info WHERE age = 22; +----+-------+ | id | name1 | +----+-------+ | 1 | aaa | | 2 | bbb | +----+-------+ 2 rows in set (0.02 sec)
结论:起别名以最开始出现的列别名为准。文档
数据类型不一致进行union 表中id是int的 name 是varchar类型的。get
mysql> select id FROM user_info WHERE age = 11 UNION ALL SELECT name FROM user_info WHERE age = 22; +-----+ | id | +-----+ | 1 | | bbb | +-----+ 2 rows in set (0.02 sec)
结论:不一样数据类型的能够合并为1列。但业务层返回数据接收处理时可能会出问题。建议数据列最好一致。it
总结:union数据会去重,union all 数据不会去重,不一样数据类型cloumn能够合并为1列。io