Oracle Schema Objects——Tables——Oracle Data Types

VARCHAR2  

CHAR

The VARCHAR2 data type stores variable-length character literals.

VARCHAR2 数据类型存储变长字符文本。

The terms literal and constant value are synonymous and refer to a fixed data value.

文本和常量值这两个术语是同义词，指的是一个固定的数据值

CHAR stores fixed-length character strings.

CHAR 存储固定长度的字符串

 VARCHAR2(25):表示该列中存储的名字最多可以长达25个字节

• When you create a table with a CHAR column, the column requires a string length. The default is 1 byte.

当创建一个有CHAR 列的表时，需要为该列指定一个字符串长度。默认值是 1 个字节。

• The database uses blanks to pad the value to the specified length.

数据库使用空格填充到指定的长度的值。

NVARCHAR2  

NCHAR

NVARCHAR2 和 NCHAR 数据类型存储 Unicode 字符数据。

Unicode 是一种通用的编码字符集，可以将任何语言的信息存储在一种字符集中。

当您创建数据库时，指定国家字符集。

NCHAR 和 NVARCHAR2 数据类型的字符集必须是 AL16UTF16 或 UTF8。

这两种字符集都使用 Unicode 编码。

当您创建一个有 NCHAR 或 NVARCHAR2 列的表时，其最大大小始终是字符长度语义。

对于 NCHAR 或 NVARCHAR2，字符长度语义是默认的，也是唯一的长度语义。

The NCHAR and NVARCHAR2 data types store Unicode character data.

Unicode is a universal encoded character set that can store information in any language using a single character set.

NCHAR stores fixed-length character strings that correspond to the national character set, whereas NVARCHAR2 stores variable length character strings.

You specify a national character set when creating a database.

The character set of NCHAR and NVARCHAR2 data types must be either AL16UTF16 or UTF8.

Both character sets use Unicode encoding.

When you create a table with an NCHAR or NVARCHAR2 column, the maximum size is always in character length semantics.

Character length semantics is the default and only length semantics for NCHAR or NVARCHAR2.

NVARCHAR2 存储可变长度的字符串

NCHAR存储对应于国家字符集的固定长度字符串

精度

Precision

he precision specifies the total number of digits.

精度指定数字的总长度。

If a precision is not specified, then the column stores the values exactly as provided by the application without any rounding.

如果不指定精度，则列按应用程序提供的数据存储值，不作任何舍入。

小数位数

Scale

The scale specifies the number of digits from the decimal point to the least significant digit.

小数位数指从十进制的小数点到最小有效数字的位数。

Positive scale counts digits to the right of the decimal point up to and including the least significant digit.

正小数位数从小数点向右计数直至最小有效位。

Negative scale counts digits to the left of the decimal point up to but not including the least significant digit.

负小数位数从小数点向左计数直至（但不包括）最小有效数字。

 If you specify a precision without a scale, as in NUMBER(6), then the scale is 0.

如果您指定了精度但没有指定小数位数，如 NUMBER(6)，那么小数位数为 0。

DATE Data Type

The DATE data type stores date and time. Although datetimes can be represented in character or number data types, DATE has special associated properties.

DATE 数据类型存储日期和时间。尽管日期时间可以用字符或数字数据类型表示，DATE 具有特殊的相关属性。

The database stores dates internally as numbers. Dates are stored in fixed-length fields of 7 bytes each, corresponding to century, year, month, day, hour, minute, and second.

数据库在内部将日期存储为数字。日期被存储为固定长度的域，共 7 个字节，分别对应世纪、 年、 月、 日、 小时、分、和秒。

Note:

• The database displays dates according to the specified format model.

数据库根据指定的 格式模型显示日期。

• A format model is a character literal that describes the format of a datetime in a character string. The standard date format is DD-MON-RR, which displays dates in the form 01-JAN-09.

格式模型是一个描述在字符串中的日期时间格式的字符文字。标准的日期格式是 DD-MON-RR，它会以 01-JAN-09 的形式显示日期。

• RR is similar to YY (the last two digits of the year), but the century of the return value varies according to the specified two-digit year and the last two digits of the current year.

RR类似于 YY （年的最后两位），但返回值的世纪值会因指定的两位年份及本年度的最后两位不同而不同

• Assume that in 1999 the database displays 01-JAN-09. If the date format uses RR, then 09 specifies 2009, whereas if the format uses YY, then 09 specifies 1909. You can change the default date format at both the instance and the session level.

假设在 1999 年，数据库显示为 01-JAN-09。如果日期格式使用 RR，那么 09 指的是 2009，而如果格式使用 YY，那么 09 指的是 1909。您可以更改实例或会话级别的默认日期格式

• Oracle Database stores time in 24-hour format—HH:MI:SS. If no time portion is entered, then by default the time in a date field is 00:00:00 A.M. In a time-only entry, the date portion defaults to the first day of the current month.

Oracle 数据库以 24 小时制格式存储时间 — — HH:MI:SS。如果不输入的任何时间部分，那么默认情况下，日期字段中的时间是凌晨 00: 00: 00，如果只输入了时间，则日期部分默认为当月的第一天。

TIMESTAMP Data Type

• The TIMESTAMP data type is an extension of the DATE data type.

TIMESTAMP 数据类型是日期数据类型的扩展

• It stores fractional seconds in addition to the information stored in the DATE data type.

除了存储在 DATE 数据类型的信息，它同时还存储秒的小数部分。

• The TIMESTAMP data type is useful for storing precise time values, such as in applications that must track event order.

TIMESTAMP 数据类型对于存储精确时间的值很有用，比如那些必须跟踪事件顺序的应用程序。

• The DATETIME data types TIMESTAMP WITH TIME ZONE and TIMESTAMP WITH LOCAL TIME ZONE are time-zone aware.

DATETIME 数据类型 TIMESTAMP WITH TIME ZONE 和TIMESTAMP WITH LOCAL TIME ZONE 能感知时区。

• When a user selects the data, the value is adjusted to the time zone of the user session. This data type is useful for collecting and evaluating date information across geographic regions.