如何更改 Oracle Select 中的日期类型？
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How to change the date type in a Oracle Select?
提问by André
I'm usualy use PostgreSQL, but I'm currently doing it in Oracle.
我通常使用 PostgreSQL，但我目前在 Oracle 中使用它。
I need to chage the data type of a column in a query(select), in PostgreSQL I usualy do it in this way:
我需要在查询（选择）中更改列的数据类型，在 PostgreSQL 中我通常这样做：
select 1::varchar from table
Hoe can I do this in Oracle?
我可以在 Oracle 中执行此操作吗？
Best Regards,
此致，
回答by Michael Pakhantsov
convert to varchar
转换为 varchar
select to_char(Field) from table
truncate varchar
截断 varchar
select substr(field, 1, 1) from table
回答by Allan
As @Michael Pakhantsov points out, to_charworks for converting to string. Likewise, to_dateand to_timestampare the standard when converting strings to dates and timestamps respectively. However, if you find that you need to perform a more exotic conversion (varchar2 to raw, for instance), then castis your friend:
正如@Michael Pakhantsov 指出的，to_char适用于转换为字符串。同样，to_date和to_timestamp分别是将字符串转换为日期和时间戳时的标准。但是，如果您发现需要执行更奇特的转换（例如，从 varchar2 到 raw），那么cast您的朋友：
Number to string:
数字到字符串：
select cast(field as varchar2(30)) from table;
String to Raw:
字符串到原始：
select cast(field as raw(16)) from table;
回答by mach128x
Like @Allan I like to use cast(), as it can go from and to many types.
However, as the official documentation says,
像@Allan 一样，我喜欢使用cast()，因为它可以转换为多种类型。
但是，正如官方文档所说，
CAST does not support LONG, LONG RAW, any of the LOB datatypes, or the Oracle-supplied types
CAST 不支持 LONG、LONG RAW、任何 LOB 数据类型或 Oracle 提供的类型
So it cannot be used to convert to CLOB, for example, Oracle will throw
所以不能用来转换为CLOB，比如Oracle会抛出
ORA-00932: inconsistent datatypes: expected - got CLOB
ORA-00932: 不一致的数据类型：预期 - 得到 CLOB
