【Python】sql-内连接，左连接，右连接，union

内连接：

mysql> select * from book_wangjing as book_1 inner join user_wangjing as user_1 on book_1.id=user_1.id limit 2;

+----+------------------------+-------------+-------+--------------+----+---------------------+------------+---------------------+----------+

| id | book_name              | book_author | price | publish_date | id | date1               | date2      | date3               | time     |

+----+------------------------+-------------+-------+--------------+----+---------------------+------------+---------------------+----------+

|  1 | webdriver 框架实战指南 | 吴老师      |   100 | 2018-05-05   |  1 | 2018-05-05 00:08:24 | 2018-05-05 | 2018-05-05 00:08:24 | 00:08:24 |

|  2 | how google test        | 李老师      |    99 | 2018-05-05   |  2 | 2018-05-05 00:08:54 | 2018-05-05 | 2018-05-05 00:08:54 | 00:08:54 |

+----+------------------------+-------------+-------+--------------+----+---------------------+------------+---------------------+----------+

2 rows in set (0.00 sec)

 

左连接：

mysql> select a.* from author_wangjing as a left join book_wangjing as b on a.author_name=b.book_author;

+----+-------------+--------+

| id | author_name | salary |

+----+-------------+--------+

|  1 | 吴老师      |  10000 |

|  3 | Mr Jackson  |  15000 |

|  2 | 张老师      |  13000 |

+----+-------------+--------+

3 rows in set (0.07 sec)

 

右连接：

mysql> select * from user_wangjing as user_1 right join book_wangjing as book_1 on user_1.id=book_1.id;

+------+---------------------+------------+---------------------+----------+----+------------------------+-------------+-------+--------------+

| id   | date1               | date2      | date3               | time     | id | book_name              | book_author | price | publish_date |

+------+---------------------+------------+---------------------+----------+----+------------------------+-------------+-------+--------------+

|    1 | 2018-05-05 00:08:24 | 2018-05-05 | 2018-05-05 00:08:24 | 00:08:24 |  1 | webdriver 框架实战指南 | 吴老师      |   100 | 2018-05-05   |

|    2 | 2018-05-05 00:08:54 | 2018-05-05 | 2018-05-05 00:08:54 | 00:08:54 |  2 | how google test        | 李老师      |    99 | 2018-05-05   |

|    3 | 2018-05-05 00:08:55 | 2018-05-05 | 2018-05-05 00:08:55 | 00:08:55 |  3 | unit test              | 李老师      |     1 | 2018-05-05   |

|    4 | 2018-05-05 00:08:56 | 2018-05-05 | 2018-05-05 00:08:56 | 00:08:56 |  4 | function test          | 李老师      |    10 | 2017-11-10   |

| NULL | NULL                | NULL       | NULL                | NULL     |  5 | function test          | Mr Jackson  |    10 | 2013-06-10   |

+------+---------------------+------------+---------------------+----------+----+------------------------+-------------+-------+--------------+

5 rows in set (0.07 sec)

 

mysql> select a.* from author_wangjing as a right join book_wangjing as b on a.author_name=b.book_author;

+------+-------------+--------+

| id   | author_name | salary |

+------+-------------+--------+

|    1 | 吴老师      |  10000 |

|    3 | Mr Jackson  |  15000 |

| NULL | NULL        |   NULL |

| NULL | NULL        |   NULL |

| NULL | NULL        |   NULL |

+------+-------------+--------+

 

union：

mysql> select book_author from book_wangjing union select author_name from author_wangjing;

+-------------+

| book_author |

+-------------+

| 吴老师      |

| 李老师      |

| Mr Jackson  |

| 张老师      |

+-------------+

4 rows in set (0.06 sec)

 

1、     内连接：结果集中出现的a.author_name和b.book_author必须同时在两个表存在

2、     左连接：结果集中出现的a.author_name必须在左表(author_wangjing表)存在且全部显示

3、     右连接：结果集中出现的b.book_author必须在右表(book_wangjing表)存在且全部显示

4、 union：做合并，使用union时左表的数据列要和右表的数据列一样多