联合多表查询

菜鸟教程join
日常应用较多的是从多个表格中获取数据。使用join可以在多个表查询进行select、update、delete。
join按照功能分为三类：

inner join(内连接、或等值连接)：获取两个表中字段匹配关系的记录
left join(左连接)：获取左表所有记录，即使右表没有对应匹配的记录
right join(右连接)：获取右表所有记录，即使左表没有对应匹配的记录。

如果只写join默认inner join

1.内连接

select a.runoob_id,a.runoob_author,b.runoob_count from 
    runoob_tbl a join tcount_tbl b 
    on a.runoob_author = b.runoob_author;
-- 等价于
select a.runoob_id,a.runoob_author,b.runoob_count from 
    runoob_tbl a,  tcount_tbl b 
    where a.runoob_author = b.runoob_author;

2.左连接

SELECT a.runoob_id, a.runoob_author, b.runoob_count 
    FROM runoob_tbl a LEFT JOIN tcount_tbl b 
    ON a.runoob_author = b.runoob_author;

3.右连接

``sql
SELECT a.runoob_id, a.runoob_author, b.runoob_count
FROM runoob_tbl a RIGHT JOIN tcount_tbl b
ON a.runoob_author = b.runoob_author;
```

4.内连接、左连接、右连接区别总结：

以上示例的准备数据，来自菜鸟

SET NAMES utf8;
SET FOREIGN_KEY_CHECKS = 0;
--  创建表结构`runoob_tbl`
DROP TABLE IF EXISTS `runoob_tbl`;
CREATE TABLE `runoob_tbl` (
  `runoob_id` int(11) NOT NULL AUTO_INCREMENT,
  `runoob_title` varchar(100) NOT NULL,
  `runoob_author` varchar(40) NOT NULL,
  `submission_date` date DEFAULT NULL,
  PRIMARY KEY (`runoob_id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;

--  插入数据
INSERT INTO `runoob_tbl` VALUES 
    ('1', '学习 PHP', '菜鸟教程', '2017-04-12'), 
    ('2', '学习 MySQL', '菜鸟教程', '2017-04-12'), 
    ('3', '学习 Java', 'RUNOOB.COM', '2015-05-01'), 
    ('4', '学习 Python', 'RUNOOB.COM', '2016-03-06'), 
    ('5', '学习 C', 'FK', '2017-04-05');
COMMIT;

--  创建表结构 `tcount_tbl`
DROP TABLE IF EXISTS `tcount_tbl`;
CREATE TABLE `tcount_tbl` (
  `runoob_author` varchar(255) NOT NULL DEFAULT '',
  `runoob_count` int(11) NOT NULL DEFAULT '0'
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

--  插入数据 `tcount_tbl`
INSERT INTO `tcount_tbl` VALUES 
    ('菜鸟教程', '10'), 
    ('RUNOOB.COM ', '20'), 
    ('Google', '22');
COMMIT;

SET FOREIGN_KEY_CHECKS = 1;

mysql-4连接