mysql 面试基础操作

• 建表

  • 用户表

  CREATE TABLE `user` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(32) DEFAULT NULL,
  `age` tinyint(4) DEFAULT NULL,
  `sex` char(4) DEFAULT NULL,
  PRIMARY KEY (`id`)
  ) ENGINE=MyISAM DEFAULT CHARSET=utf8

  • 学生表

  CREATE TABLE `student` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `user_id` int(11) DEFAULT NULL,
  `major` varchar(64) DEFAULT NULL,
  `class` varchar(64) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `unique_user_id` (`user_id`)
  ) ENGINE=MyISAM DEFAULT CHARSET=utf8

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• 插入数据

insert into user(name,age,sex) values('张三',18,'男');
insert into user(name,age,sex) values('李四',19,'女');
insert into user(name,age,sex) values('王五',18,'女');
insert into user(name,age,sex) values('赵六',20,'男');
insert into user(name,age,sex) values('李四',20,'男');

insert into student(user_id,major,class) values(2,'计算机','计算机151班');
insert into student(user_id,major,class) values(1,'机电','机电151班');
insert into student(user_id,major,class) values(3,'商学院','商学151班');
insert into student(user_id,major,class) values(4,'经管','经管151班');

• 查找数据库字段中重复的数据

 SELECT name FROM user GROUP BY name HAVING COUNT(*)>1;

• union 和 union all 的使用和区别

  • union 和union all 都可以将多个 select 查询组合成一个结果集
  • union 和union all 组合的 select 查询的字段数必须一样
  • 如下一个查询 * 一个只只查询 name 就会报错

  mysql> select * from user union  all select name from user where name='李四';
  ERROR 1222 (21000): The used SELECT statements have a different number of columns这里写代码片

  • union 会过滤相同的数据，如下

  mysql> select name from user union select name from user where name='李四';
  +--------+
  | name   |
  +--------+
  | 张三   |
  | 李四   |
  | 王五   |
  | 赵六   |
  | 刘七   |
  | 宋八   |
  +--------+
  6 rows in set (0.00 sec)
  

  • union all 不会过滤相同的数据

  mysql> select name from user union all select name from user where name='李四';
  +--------+
  | name   |
  +--------+
  | 张三   |
  | 李四   |
  | 王五   |
  | 赵六   |
  | 李四   |
  | 刘七   |
  | 宋八   |
  | 李四   |
  | 李四   |
  +--------+
  9 rows in set (0.00 sec)