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原文链接：https://blog.csdn.net/qq_30038111/article/details/79740391

表：A、B
A 字段：id，name
B 字段：id，a_id
关键名词：主表、关联表、关联条件、筛选条件
例子：

# 主表：A、关联表：B、关联条件：A.od=B.a_id、筛选条件：B.id=1
A left join B on A.id=B.a_id and B.id=1

结论：

表 A 和表 B 的连接依靠关联条件
主表的筛选条件，应该放置在 where 条件后
on 后面的筛选条件是针对于关联表
关联表的筛选条件，如果放置在 on 后面，那么 A 和 B 的连接顺序为：B 表先按条件查询，再与 A 表连接，即先筛选再连接；如果放置在 where 后面，那么 A 和 B 的连接顺序为：A 与 B 连接后，再从连接表中筛选，即先连接再筛选
where 后面的条件是对连接后的数据进行筛选
如果存在多个left join on，请注意on后面的条件与哪个表关联。这一条统计的SQL很重要！例如表A，B，C，A left join B on A.x = B.x left join C on A.x = C.x，B和C的都要和A建立关联，B和C之间是没有任何数据上的关系。但是 如果把A.x = C.x改成B.x = C.x，那么B和C的表数据先建立关联并过滤数据，再与A表数据进行关联，这样可能会出现数据丢失！
验证结论：

主表的筛选条件应该放置在 where 条件后，如果放在 on 后面，对主表的查询没有意义

# 主表的筛选条件在 on 后
SELECT * from 
    edu_student est
left JOIN edu_score ese ON est.stu_id = ese.stu_id and est.stu_name='盲僧'
# 区别于
# 主表的筛选条件在 where 后
SELECT * from 
    edu_student est
left JOIN edu_score ese ON est.stu_id = ese.stu_id where est.stu_name='盲僧'

# 关联表的筛选条件放在 on 后面
# 关联表先筛选再连接
SELECT * from 
    edu_student est
left JOIN edu_score ese ON est.stu_id = ese.stu_id and ese.scores > 90
# 等价于
SELECT * FROM edu_student est LEFT JOIN ( SELECT * FROM edu_score ese WHERE ese.scores > 90 ) a ON est.stu_id = a.stu_id

# 关联表的筛选条件放在 where 后
# 主表、关联表先连接，再筛选
SELECT * from 
    edu_student est
left JOIN edu_score ese ON est.stu_id = ese.stu_id where ese.scores > 90

CREATE TABLE `edu_student` (
  `stu_id` varchar(16) NOT NULL COMMENT '学号',
  `stu_name` varchar(20) NOT NULL COMMENT '学生姓名',
  PRIMARY KEY (`stu_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='学生表';

-- ----------------------------
-- Records of edu_student
-- ----------------------------
INSERT INTO `edu_student` VALUES ('1001', '盲僧');
INSERT INTO `edu_student` VALUES ('1002', '赵信');
INSERT INTO `edu_student` VALUES ('1003', '皇子');
INSERT INTO `edu_student` VALUES ('1004', '寒冰');
INSERT INTO `edu_student` VALUES ('1005', '蛮王');
INSERT INTO `edu_student` VALUES ('1006', '狐狸');

CREATE TABLE `edu_score` (
  `stu_id` varchar(16) NOT NULL COMMENT '学号',
  `course_no` varchar(20) NOT NULL COMMENT '课程编号',
  `scores` float DEFAULT NULL COMMENT '得分',
  PRIMARY KEY (`stu_id`,`course_no`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='成绩表';

-- ----------------------------
-- Records of edu_score
-- ----------------------------
INSERT INTO `edu_score` VALUES ('1001', 'C001', '67');
INSERT INTO `edu_score` VALUES ('1001', 'C002', '87');
INSERT INTO `edu_score` VALUES ('1001', 'C003', '83');
INSERT INTO `edu_score` VALUES ('1001', 'C004', '88');
INSERT INTO `edu_score` VALUES ('1001', 'C005', '77');
INSERT INTO `edu_score` VALUES ('1002', 'C001', '68');
INSERT INTO `edu_score` VALUES ('1002', 'C002', '88');
INSERT INTO `edu_score` VALUES ('1002', 'C003', '84');
INSERT INTO `edu_score` VALUES ('1002', 'C004', '89');
INSERT INTO `edu_score` VALUES ('1002', 'C005', '78');
INSERT INTO `edu_score` VALUES ('1003', 'C001', '69');
INSERT INTO `edu_score` VALUES ('1003', 'C002', '89');
INSERT INTO `edu_score` VALUES ('1003', 'C003', '85');
INSERT INTO `edu_score` VALUES ('1003', 'C004', '90');
INSERT INTO `edu_score` VALUES ('1003', 'C005', '79');
INSERT INTO `edu_score` VALUES ('1004', 'C001', '70');
INSERT INTO `edu_score` VALUES ('1004', 'C002', '90');
INSERT INTO `edu_score` VALUES ('1004', 'C003', '86');
INSERT INTO `edu_score` VALUES ('1004', 'C004', '91');
INSERT INTO `edu_score` VALUES ('1005', 'C001', '71');
INSERT INTO `edu_score` VALUES ('1005', 'C002', '91');
INSERT INTO `edu_score` VALUES ('1005', 'C003', '87');
INSERT INTO `edu_score` VALUES ('1005', 'C004', '92');
INSERT INTO `edu_score` VALUES ('1006', 'C001', '72');
INSERT INTO `edu_score` VALUES ('1006', 'C002', '92');
INSERT INTO `edu_score` VALUES ('1006', 'C003', '88');
INSERT INTO `edu_score` VALUES ('1006', 'C004', '93');