mysql 測試left join、right join、inner join用法

原創

2020-02-21 04:06

CREATE TABLE `sdb_b2c_te` (
  `id` int(2) unsigned NOT NULL AUTO_INCREMENT,
  `order_id` int(11) NOT NULL DEFAULT '0',
  `ship_name` varchar(50) NOT NULL DEFAULT '',
  `age` int(11) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO `sdb_b2c_te` (`id`, `order_id`, `ship_name`, `age`)
VALUES
	(1, 1, 'zhangsan', 10),
	(2, 2, 'lisi', 10),
	(3, 3, 'wangwu', 10),
	(4, 5, 'kobe', 40);
	
CREATE TABLE `sdb_b2c_te1` (
  `id` int(2) unsigned NOT NULL AUTO_INCREMENT,
  `order_id` int(11) NOT NULL DEFAULT '0',
  `goods_id` int(11) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO `sdb_b2c_te1` (`id`, `order_id`, `goods_id`)
VALUES
	(1, 1, 100),
	(2, 2, 200),
	(3, 5, 100),
	(4, 4, 100);

測試inner join：

select sdb_b2c_te.id as 'sdb_b2c_te.id',sdb_b2c_te.age as 'sdb_b2c_te.age',sdb_b2c_te1.id as 'sdb_b2c_te1.id',sdb_b2c_te1.goods_id as 'sdb_b2c_te1.goods_id' from sdb_b2c_te inner join sdb_b2c_te1 on sdb_b2c_te.order_id=sdb_b2c_te1.order_id

總結：inner join取的是兩個表的交集，如：a inner join b on a.order_id=b.order_id，取的是a表和b表中order_id相等的數據，如果有where條件，就先過濾where條件，然後在過濾on，然後取過濾後的數據的交集部分。

測試left join：有where條件

select sdb_b2c_te.id as 'sdb_b2c_te.id',sdb_b2c_te.age as 'sdb_b2c_te.age',sdb_b2c_te1.id as 'sdb_b2c_te1.id',sdb_b2c_te1.goods_id as 'sdb_b2c_te1.goods_id' from sdb_b2c_te left join sdb_b2c_te1 on sdb_b2c_te.order_id=sdb_b2c_te1.order_id 
where sdb_b2c_te.age = 10

總結：如果有where條件，則先過濾where條件，然後從過濾後的數據中取出所有主表(left join左邊的表)中的數據，然後在根據on條件去輔表(left join右邊)找數據，如果有，則直接取出數據，如果沒有，則把輔表相關字段置爲null。

測試right join：不加where條件

select sdb_b2c_te.id as 'sdb_b2c_te.id',sdb_b2c_te.age as 'sdb_b2c_te.age',sdb_b2c_te1.id as 'sdb_b2c_te1.id',sdb_b2c_te1.goods_id as 'sdb_b2c_te1.goods_id' from sdb_b2c_te left join sdb_b2c_te1 on sdb_b2c_te.order_id=sdb_b2c_te1.order_id 

總結：沒有where條件，則先取出主表中的所有數據，然後在根據on條件去輔表查數據，有，則直接取出，沒有，則把輔表相關字段置爲null。