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.net/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.net/limingchuan123456789/article/details/8754273

• UNION语句后可以加ORDER BY来针对选出后的内容排序