SQL语句七种JOINS理论及实践

七种SQL JOINS

一、建表语句

部门表：

DROP TABLE IF EXISTS `dept`;
CREATE TABLE `dept` (
	`dept_id` int(11) NOT NULL AUTO_INCREMENT,
    `dept_name` varchar(30) DEFAULT NULL,
    `dept_number` int(11) DEFAULT NULL,
    PRIMARY KEY (`dept_id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

INSERT INTO `dept` VALUES ('1', 'AA', '100');
INSERT INTO `dept` VALUES ('2', 'BB', '200');
INSERT INTO `dept` VALUES ('3', 'CC', '300');
INSERT INTO `dept` VALUES ('4', 'DD', '400');
INSERT INTO `dept` VALUES ('5', 'HH', '500');

员工表：

DROP TABLE IF EXISTS `emp`;
CREATE TABLE `emp` (
	`emp_id` int(11) NOT NULL AUTO_INCREMENT,
    `emp_name` varchar(30) DEFAULT NULL,
    `emp_age` int(11) DEFAULT NULL,
    `dept_id` int(11) NOT NULL,
    PRIMARY KEY (`emp_id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

INSERT INTO `emp` VALUES('1', 'zhangsan', '20', '1');
INSERT INTO `emp` VALUES('2', 'lisi', '25', '6');
INSERT INTO `emp` VALUES('3', 'wangwu', '19', '4');
INSERT INTO `emp` VALUES('4', 'zhaoliu', '29', '5');
INSERT INTO `emp` VALUES('5', 'xiaohong', '30', '2');
INSERT INTO `emp` VALUES('6', 'xiaohu', '26', '3');
INSERT INTO `emp` VALUES('7', 'zhangle', '23', '3');
INSERT INTO `emp` VALUES('8', 'qingtian', '38', '3');
INSERT INTO `emp` VALUES('9', 'xiayutian', '36', '2');
INSERT INTO `emp` VALUES('10', 'fangjia', '40', '1');

二、情景分析

下面针对这一情景练习七种SQL JOINS

1. 左连接(left join)

select <select_list> FROM Table A LEFT JOIN Table B ON A.key = B.key;

2. 右连接(right join)

select <select_list> FROM Table A RIGHT JOIN Table B ON A.key = B.key;

3. 内连接(inner join)

select <select_list> FROM Table A INNER JOIN Table B ON A.key = B.key;

4. 左外连接(left join且右表=null)

select <select_list> FROM Table A LEFT JOIN Table B on A.key = B.key where B.key is null;

5. 右外连接(right join且左表=null)

select <select_list> FROM Table A RIGHT JOIN Table B on A.key = b.key where A.key is null;

6. 全连接(full outer join)

select <select_list> FROM Table A FULL OUTER JOIN Table B on A.key = B.key;

MySQL不支持FULL OUTER JOIN(在ORACLE支持)。因此使用UNION的方式来实现

select <select_list> FROM Table A LEFT JOIN Table B on A.key = B.key UNION select <select_list> FROM Table A RIGHT JOIN Table B on A.key = B.key;

7. 全外连接(full outer join且左右表=null)

select <select_list> FROM Table A FULL OUTER JOIN Table B on A.key = B.key where A.key is null or B.key is null;

MySQL不支持FULL OUTER JOIN(在ORACLE支持)。因此使用UNION的方式来实现

select <select_list> FROM Table A LEFT JOIN Table B on A.key = B.key where B.key is null UNION select <select_list> FROM Table A RIGHT JOIN Table B on A.key = B.key where A.key is null;