为什么 CAST 会截断文本而不是抛出错误？
Why does CAST truncate text instead of throwing an error?
将文本转换为比值短的长度会导致文本被截断。
select
cast('ROAD-1234' as varchar2(8)) as id
from
dual
ID
--------
ROAD-123
--^ Notice that the number 4 has been removed
我原以为 CAST AS VARCHAR2 的行为与 CAST AS NUMBER 类似。
select
cast(1234 as number(3)) as id
from
dual
Error: ORA-01438: value larger than specified precision allowed for this column
为什么 CAST AS VARCHAR2 会静默截断文本而不是像 CAST AS NUMBER 那样抛出错误？
这是 ANSI SQL 标准的一部分，其他符合 ANSI SQL 的 RDBMS 也做同样的事情：
For fixed length character string targets, if the length of the source equals the fixed length of the target, the result of the CAST is the source string. If the length of the source is shorter than the fixed length of the target, the result of the CAST is the source string padded on the right with however many spaces are required to make the lengths match. If the length of the source is longer than the fixed length of the target, the result of the CAST is a character string that contains as much of the source string as possible – in this case, if the truncated characters are not all spaces, your DBMS will return the SQLSTATE warning 01004 "warning-string data, right truncation".
不幸的是，数据库通常无法同时发出警告和 return 结果，因此它们会静默截断结果
将文本转换为比值短的长度会导致文本被截断。
select
cast('ROAD-1234' as varchar2(8)) as id
from
dual
ID
--------
ROAD-123
--^ Notice that the number 4 has been removed
我原以为 CAST AS VARCHAR2 的行为与 CAST AS NUMBER 类似。
select
cast(1234 as number(3)) as id
from
dual
Error: ORA-01438: value larger than specified precision allowed for this column
为什么 CAST AS VARCHAR2 会静默截断文本而不是像 CAST AS NUMBER 那样抛出错误？
这是 ANSI SQL 标准的一部分，其他符合 ANSI SQL 的 RDBMS 也做同样的事情：
For fixed length character string targets, if the length of the source equals the fixed length of the target, the result of the CAST is the source string. If the length of the source is shorter than the fixed length of the target, the result of the CAST is the source string padded on the right with however many spaces are required to make the lengths match. If the length of the source is longer than the fixed length of the target, the result of the CAST is a character string that contains as much of the source string as possible – in this case, if the truncated characters are not all spaces, your DBMS will return the SQLSTATE warning 01004 "warning-string data, right truncation".
不幸的是，数据库通常无法同时发出警告和 return 结果，因此它们会静默截断结果