原文：http://www.cnblogs.com/xwdreamer/archive/2010/12/15/2297058.html
内连接：把两个表中数据对应的数据查出来
外连接：以某个表为基础把对应数据查出来
首先创建数据库中的表，数据库代码如下：
/*
Navicat MySQL Data Transfer
Source Server : localhost_3306
Source Server Version : 50150
Source Host : localhost:3306
Source Database : store
Target Server Type : MYSQL
Target Server Version : 50150
File Encoding : 65001
Date: 2010-12-15 16:27:53
*/
SET FOREIGN_KEY_CHECKS=0;
-- ----------------------------
-- Table structure for `grade`
-- ----------------------------
DROP TABLE IF EXISTS `grade`;
CREATE TABLE `grade` (
`no` int(11) NOT NULL AUTO_INCREMENT,
`grade` int(11) NOT NULL,
PRIMARY KEY (`no`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of grade
-- ----------------------------
INSERT INTO grade VALUES ('', '');
INSERT INTO grade VALUES ('', '');
INSERT INTO grade VALUES ('', '');
-- ----------------------------
-- Table structure for `student`
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
`no` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(20) NOT NULL,
PRIMARY KEY (`no`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO student VALUES ('', 'a');
INSERT INTO student VALUES ('', 'b');
INSERT INTO student VALUES ('', 'c');
INSERT INTO student VALUES ('', 'd');
student表中的字段分别是no和name，grade表中的字段是no和grade。两张表中的no都代表的是学生的学号。
查询student表的结果：
mysql> select * from grade;
+----+-------+
| no | grade |
+----+-------+
| 1 | 90 |
| 2 | 80 |
| 3 | 70 |
+----+-------+
rows in set
查询grade表的结果：
mysql> select * from student s inner join grade g on s.no=g.no; +----+------+----+-------+
| no | name | no | grade |
+----+------+----+-------+
| 1 | a | 1 | 90 |
| 2 | b | 2 | 80 |
| 3 | c | 3 | 70 |
+----+------+----+-------+
rows in set
左连接（左表中所有数据，右表中对应数据）
mysql> select * from student as s left join grade as
g on s.no=g.no;
+----+------+------+-------+
| no | name | no | grade |
+----+------+------+-------+
| 1 | a | 1 | 90 |
| 2 | b | 2 | 80 |
| 3 | c | 3 | 70 |
| 4 | d | NULL | NULL |
+----+------+------+-------+
rows in set
右连接（右表中所有数据，左表中对应数据）
mysql> select * from student as s right
join grade as g on s.no=g.no;
+----+------+----+-------+
| no | name | no | grade |
+----+------+----+-------+
| 1 | a | 1 | 90 |
| 2 | b | 2 | 80 |
| 3 | c | 3 | 70 |
+----+------+----+-------+
rows in set