3回答

• 

  慕粉1556379847

  2018-10-09 21:17:17

  按照道理来说是这样的，但是如果你左右表写反了，改左右连接会更加方便快捷。

  0 0
• 

  恶魔督督

  2018-07-02 16:08:14

  -- 注意自己是否需要建库

  create database if not exists mysqldv1;

  use mysqldv1; 

   

  -- 新建 user1 表 

  create table if not exists user1(

    id smallint unsigned primary key auto_increment, 

    user_name varchar(40), 

    over1 varchar(40)

  );

  -- 新建 user2 表

  create table if not exists user2 (

    id smallint unsigned primary key auto_increment,

    user_name varchar(40),

    over1 varchar(40)

  );

  -- 新建 user_kills 表

  create table if not exists user_kills (

    id smallint unsigned primary key auto_increment,

    user_id smallint unsigned,

    timestr timestamp default CURRENT_TIMESTAMP,

    kills smallint unsigned

  );

  -- 插入 user1 表数据

  insert into user1(user_name,over1) values ('唐僧', '旃檀功德佛');

  insert into user1(user_name,over1) values ('猪八戒', '净坛使者');

  insert into user1(user_name,over1) values ('孙悟空', '斗战胜佛');

  insert into user1(user_name,over1) values ('沙僧', '金身罗汉');

  -- 插入 user2 表数据

  insert into user2(user_name,over1) values ('孙悟空', '成佛');

  insert into user2(user_name,over1) values ('牛魔王', '被降服');

  insert into user2(user_name,over1) values ('蛟魔王', '被降服');

  insert into user2(user_name,over1) values ('鹏魔王', '被降服');

  insert into user2(user_name,over1) values ('狮驼王', '被降服');

  -- 插入 user_kills 表数据

  insert into user_kills(user_id, timestr, kills) values (2, timestamp('2013-01-10'), 10);

  insert into user_kills(user_id, timestr, kills) values (2, timestamp('2013-02-01'), 2);

  insert into user_kills(user_id, timestr, kills) values (2, timestamp('2013-02-05'), 12);

  insert into user_kills(user_id, timestr, kills) values (4, timestamp('2013-01-10'), 3);

  insert into user_kills(user_id, timestr, kills) values (4, timestamp('2013-02-11'), 5);

  insert into user_kills(user_id, timestr, kills) values (2, timestamp('2013-02-06'), 1);

  insert into user_kills(user_id, timestr, kills) values (3, timestamp('2013-01-11'), 20);

  insert into user_kills(user_id, timestr, kills) values (2, timestamp('2013-02-12'), 10);

  insert into user_kills(user_id, timestr, kills) values (3, timestamp('2013-02-07'), 17);

  
  

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• 

  周文春

  2018-04-20 17:46:15

    LEFT JOIN 关键字会从左表 (table_name1) 那里返回所有的行，即使在右表 (table_name2) 中没有匹配的行。

            RIGHT JOIN 关键字会右表 (table_name2) 那里返回所有的行，即使在左表 (table_name1) 中没有匹配的行。

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