mysql+union+不存在时,mysql union用法以及注意事项

句式：SELECT field1,field2... FROM table1 UNION SELECT field3,field4... FROM table2;

注意：

前后所选字段数目需相同，否则报错，例：

mysql> select `submission_date` from `runoob_tbl` union all select `runoob_author`,`submission_date` from `runoob_tbl2`;

1222 - The used SELECT statements have a different number of columns

所选出的内容显示时，会以前面的字段名称命名：

mysql> select `runoob_id`,`runoob_title` from `runoob_tbl` union select `runoob_author`,`submission_date` from `runoob_tbl2`;

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

| runoob_id | runoob_title |

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

| 1 | 学习 PHP |

| 2 | 学习 MySQL |

| 3 | 学习 C++ |

| 4 | JAVA 教程 |

| 5 | 连接 |

| 菜鸟教程 | 2018-08-15 |

| 菜鸟教程 | 2018-08-15 |

| RUNOOB.COM | 2016-05-06 |

| RUNOOB.COM | 2016-03-06 |

| 随便 | 2018-08-18 |

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

UNION ALL 表示将包括重的内容一起显示出来 ，UNION 或UNION DSTINCT表示去重显示。

mysql> select `runoob_author`,`submission_date` from `runoob_tbl` union select `runoob_author`,`submission_date` from `runoob_tbl2`;

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

| runoob_author | submission_date |

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

| 爆炸菜鸟教程爆炸 | 2018-08-15 |

| 菜鸟教程 | 2018-08-15 |

| RUNOOB.COM | 2016-05-06 |

| RUNOOB.COM | 2016-03-06 |

| 随便写 | 2018-08-18 |

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

5 rows in set

mysql> select `runoob_author`,`submission_date` from `runoob_tbl` union all

select `runoob_author`,`submission_date` from `runoob_tbl2`;

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

| runoob_author | submission_date |

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

| 爆炸菜鸟教程爆炸 | 2018-08-15 |

| 菜鸟教程 | 2018-08-15 |

| RUNOOB.COM | 2016-05-06 |

| RUNOOB.COM | 2016-03-06 |

| 随便写 | 2018-08-18 |

| 爆炸菜鸟教程爆炸 | 2018-08-15 |

| 菜鸟教程 | 2018-08-15 |

| RUNOOB.COM | 2016-05-06 |

| RUNOOB.COM | 2016-03-06 |

| 随便写 | 2018-08-18 |

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

其他可参考：https://blog.csdn/limingchuan123456789/article/details/8754273

待深究问题：UNION选出后的结果数据类型是怎么判定的？了解数据类型优先级：https://blog.csdn/limingchuan123456789/article/details/8754273

UNION语句后可以加ORDER BY来针对选出后的内容排序