在 sql 中乘以两个 varchars

Question

这是我尝试过的方法，但出现错误：

MariaDB [test]> create table table1 (length varchar, breadth varchar);                                                             
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ' breadth varchar)' at line 1
MariaDB [test]> create table table1 (length varchar(20), breadth varchar(20));                                                         
Query OK, 0 rows affected (0.06 sec)
MariaDB [test]> insert into table1 (length, breadth) values ('12','11');                                                                                                                               
Query OK, 1 row affected (0.05 sec)                                                                                                                                                                    

MariaDB [test]> select * from table1;                                                                                                                                                                  
+--------+---------+                                                                                                                                                                                   
| length | breadth |                                                                                                                                                                                   
+--------+---------+                                                                                                                                                                                   
| 12     | 11      |                                                                                                                                                                                   
+--------+---------+                                                                                                                                                                                   
1 row in set (0.02 sec)                                                                                                                                                                                                                                                                                                                                                                                   
MariaDB [test]> select convert (int, length)*convert(int, breadth) as T from table1;                                                                                                                   
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'int, length)*convert(int, breadth) as T fr
om table1' at line 1                                                                                                                                                                                   
MariaDB [test]> select convert(int, length)*convert(int, breadth) as T from table1;                                                                                                                    
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'int, length)*convert(int, breadth) as T fr
om table1' at line 1                                                                                                                                                                                   

Answer 1

使用cast():

select cast(length as int) * cast(breadth as int) as T
from table1;  

如documentation中所述，convert()用于不同字符集之间的转换。您将 MySQL（和 MariaDB）对 convert() 的使用与 SQL 服务器的使用混淆了。

顺便说一下，您甚至不需要明确的 cast()（至少在 MySQL 中）。引擎将为您进行隐式转换：

select length * breadth as T from table1;  

虽然支持，但我并不提倡依赖隐式转换。

Here 是 SQL Fiddle.