mysql基础

Posted 2020-10-09 懒企鹅

左外连，右外连

CREATE DATABASE test_mysql;
use test_mysql;
CREATE TABLE left_tb( id INT(11) PRIMARY KEY AUTO_INCREMENT,name VARCHAR(32) );
CREATE TABLE right_tb( id INT(11) PRIMARY KEY AUTO_INCREMENT,name VARCHAR(32),age INT(11) );

INSERT INTO left_tb (name) VALUES (‘zhangsan‘);
INSERT INTO left_tb (name) VALUES (‘lisi‘);
INSERT INTO left_tb (name) VALUES (‘wangwu‘);

SELECT * from left_tb;
1	zhangsan
2	lisi
3	wangwu


INSERT INTO right_tb (name,age) VALUES (‘aaa‘,11);
INSERT INTO right_tb (name,age) VALUES (‘bbb‘,12);
INSERT INTO right_tb (name,age) VALUES (‘ccc‘,13);
INSERT INTO right_tb (name,age) VALUES (‘ddd‘,14);

SELECT * from right_tb;
1	aaa	11
2	bbb	12
3	ccc	13
4	ddd	14

 

SELECT * from left_tb l LEFT JOIN right_tb r ON l.id=r.id;
+----+----------+------+------+------+
| id | name     | id   | name | age  |
+----+----------+------+------+------+
|  1 | zhangsan |    1 | aaa  |   11 |
|  2 | lisi     |    2 | bbb  |   12 |
|  3 | wangwu   |    3 | ccc  |   13 |
+----+----------+------+------+------+


SELECT * from left_tb l RIGHT JOIN right_tb r ON l.id=r.id;
+------+----------+----+------+------+
| id   | name     | id | name | age  |
+------+----------+----+------+------+
|    1 | zhangsan |  1 | aaa  |   11 |
|    2 | lisi     |  2 | bbb  |   12 |
|    3 | wangwu   |  3 | ccc  |   13 |
| NULL | NULL     |  4 | ddd  |   14 |
+------+----------+----+------+------+


SELECT * from left_tb l LEFT JOIN right_tb r ON l.id=r.age;
+----+----------+------+------+------+
| id | name     | id   | name | age  |
+----+----------+------+------+------+
|  1 | zhangsan | NULL | NULL | NULL |
|  2 | lisi     | NULL | NULL | NULL |
|  3 | wangwu   | NULL | NULL | NULL |
+----+----------+------+------+------+

SELECT * from left_tb l LEFT JOIN right_tb r ON 1=1;
+----+----------+------+------+------+
| id | name     | id   | name | age  |
+----+----------+------+------+------+
|  1 | zhangsan |    1 | aaa  |   11 |
|  2 | lisi     |    1 | aaa  |   11 |
|  3 | wangwu   |    1 | aaa  |   11 |
|  1 | zhangsan |    2 | bbb  |   12 |
|  2 | lisi     |    2 | bbb  |   12 |
|  3 | wangwu   |    2 | bbb  |   12 |
|  1 | zhangsan |    3 | ccc  |   13 |
|  2 | lisi     |    3 | ccc  |   13 |
|  3 | wangwu   |    3 | ccc  |   13 |
|  1 | zhangsan |    4 | ddd  |   14 |
|  2 | lisi     |    4 | ddd  |   14 |
|  3 | wangwu   |    4 | ddd  |   14 |
+----+----------+------+------+------+