INSERT SELECT WHERE 从常量中插入多行

Question

我正在尝试插入多行，这些行的值不是取自现有表，而是从外部提供，以及使用 INSERT ... SELECT ... WHERE 的 where 条件。

以下查询无效：

mysql> insert into `my_table` SELECT 1 as a, 2 as b, 3 as c from dual UNION ALL SELECT 4 , 5 , 6 from dual UNION ALL SELECT 7 , 8 , 9  from dual where 1>2 ;
Query OK, 2 rows affected (0.01 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from my_table;
+---+---+------+
| a | b | c    |
+---+---+------+
| 1 | 2 |    3 |
| 4 | 5 |    6 |
+---+---+------+
2 rows in set (0.00 sec)

我想查询不插入任何行，因为where 条件为假。但是，where 子句仅适用于最后一个select，第 2 个selects 不受where 子句的影响。

如何改正？

Answer 1

只需将UNION ALL 子查询包装在子查询中，以便将WHERE 应用于整个结果集：

insert into `my_table` 
select a, b, c 
from (
   SELECT 1 as a, 2 as b, 3 as c 
   from dual 

   UNION ALL 

   SELECT 4 , 5 , 6 
   from dual 

   UNION ALL 

   SELECT 7 , 8 , 9  
   from dual ) as t
where a > b ;