left join和inner join举例

1：创建两个表

/*
MySQL Data Transfer
Source Host: localhost
Source Database: test
Target Host: localhost
Target Database: test
Date: 2015/8/19 11:49:26
*/

SET FOREIGN_KEY_CHECKS=0;
-- ----------------------------
-- Table structure for t_a
-- ----------------------------
CREATE TABLE `t_a` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

-- ----------------------------
-- Table structure for t_b
-- ----------------------------
CREATE TABLE `t_b` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

-- ----------------------------
-- Records 
-- ----------------------------
INSERT INTO `t_a` VALUES ('1', 'a');
INSERT INTO `t_a` VALUES ('2', 'b');
INSERT INTO `t_a` VALUES ('3', 'c');
INSERT INTO `t_a` VALUES ('4', 'd');


INSERT INTO `t_b` VALUES ('1', 'a');
INSERT INTO `t_b` VALUES ('2', 'b');
INSERT INTO `t_b` VALUES ('5', 'a');

2：left join

select   a.*  from  t_a   a
left  join t_b b 
on a.name = b.name

运行结果：

select   a.*,b.*  from  t_a   a
left  join t_b b 
on a.name = b.name

运行结果：

3：inner join

select   a.*  from  t_a   a
inner join t_b b 
on a.name = b.name

运行结果：

select   a.*,b.*  from  t_a   a
inner join t_b b 
on a.name = b.name

注意：

left join和inner join，假设on和where条件不唯一的话，主表会查询出反复数据。
所以在实际应用中。请确保on条件的唯一性。

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