Oracle field type and storage (i)

2000 byte in Oracle, not 2000 characters, 1 characters in Oracle may be 1 byte to 4 byte, depending on the setting of the database character set.

For the GBK character set, 128 characters in the ASCII code are stored using 1 bytes, the other characters are stored in double-byte storage, 128 characters in the ASCII code are stored using 1 bytes for the UTF8 character set, and the other is stored in two or three bytes. Four bytes are stored with very few characters;

The database has a parameter nls_length_semantics, which has two values of char and byte. The length of the character type is calculated as char, rather than in byte, which is useful when using the variable-length character set (Al32utf8), because the number of bytes that a character occupies is variable, which gives us an accurate estimate of the length of the field (byte).

1, CHAR.

Char is convenient to store the fixed-length data, the index on the Char field is more efficient, such as the definition of char (10), then regardless of whether you store data reached 10 bytes, take up 10 bytes of space, insufficient to automatically fill with spaces.

2. VARCHAR

VARCHAR stores variable-length data, but the storage efficiency is no higher than char. If the possible value of a field is not fixed length, we only know that it cannot exceed 10 characters, it is the most advantageous to define it as VARCHAR (10). The actual length of the varchar type is +1 of the actual length of its value. Why "+1"? This byte is used to hold the length that is actually used. From the space consideration, with the varchar suitable, from the efficiency consideration, uses the char to be suitable, the key is to find the tradeoff point according to the actual situation.

3. TEXT

Text stores non-Unicode data of variable length, with a maximum length of 2^31-1 (2,147,483,647) characters.

4, NCHAR, NVARCHAR, NTEXT

These three kinds of names from the first three more than the previous "N". It represents a character stored in a Unicode data type. We know that characters, the English character only need a byte storage is enough, but the number of Chinese characters, need two bytes of storage, English and Chinese characters at the same time prone to confusion, the Unicode character set is to solve the character set this incompatibility problem, all of its characters are expressed in two bytes, That is, the English character is also represented in two bytes.

5. Character set and national character set

1> Character Set
(1) used to store char, VARCHAR2, CLOB, long and other types of data
(2) to indicate such as table name, column name, and PL/SQL variable, etc.
(3) used to store SQL and PL + + program units, etc.

2> National Character Set

(1) to store nchar, NVARCHAR2, NCLOB and other types of data
(2) The national character set is essentially the additional character set chosen for Oracle, primarily to enhance the character processing capabilities of Oracle, since the nchar data type provides support for the use of fixed-length multibyte encoding in Asia, and the database character set is not. The national character set is redefined in oracle9i and can only be selected in Af16utf16 and UTF8 in Unicode encoding, and the default value is Af16utf16

Field type	Chinese description	Restriction conditions	Other instructions
CHAR	Fixed-length strings	Maximum length 2000bytes
VARCHAR2	Variable-length strings	Maximum length 4000bytes	Maximum length of index can be made 749
NCHAR	Fixed-length strings based on character set	Maximum length 2000bytes
NVARCHAR2	Variable-length strings based on character set	Maximum length 4000bytes
DATE	Date (day-month-year)	Dd-mm-yy (HH-MI-SS)	After rigorous testing, no problem of thousand worms
LONG	Super Long string	Maximum length 2G (231-1)	Enough to store a tome
RAW	Fixed-length binary data	Maximum length 2000bytes	can store multimedia image sound, etc.
LONG RAW	Variable-length binary data	Maximum length 2G	Ditto
Blob	Binary data	Maximum length 4G
Clob	Character data	Maximum length 4G
NCLOB	Character data based on the character set	Maximum length 4G
BFILE	Binary data stored outside the database	Maximum length 4G
ROWID	Unique line number recorded in the datasheet	10bytes	. ****.**** format, * is 0 or 1
Nrowid	Unique line number recorded in a Binary data table	Maximum length 4000bytes
Number (P S)	Number Type	P is the integer digit, S is the decimal digit
DECIMAL (P S)	Number Type	P is the integer digit, S is the decimal digit
INTEGER	Integer type	A small integer
FLOAT	Floating-point type	Number (38), double precision
REAL	Real type	Number (63), higher accuracy

Oracle field type and storage (i)