16、日期和时间处理函数全解析

日期和时间处理函数全解析

1. CAST 函数的使用

1.1 DATE 类型转换为字符串

在将 DATE 类型的值转换为字符串格式时，输出数据类型的长度必须正确。若长度不合适，输出值将超出范围，从而引发异常。以下是具体示例：

select CAST(sysdate as char(20)) from dual;
--> 25.03.2022 07:58:52
select CAST(sysdate as varchar(30)) from dual;
--> 25.03.2022 07:58:52
select CAST(sysdate as char(1)) from dual;
--> ORA-25137: "Data value out of range"
select CAST(sysdate as varchar(3)) from dual;
--> ORA-01801:
--> "date format is too long for internal buffer"

1.2 TIMESTAMP 类型转换为字符串

TIMESTAMP 类型转换为字符串的原则与 DATE 类型转换相同。示例如下：

select CAST(systimestamp as char(100)) from dual;
--> 25.03.2022 08:17:13,874000 +01:00
select CAST(systimestamp as varchar(