2.3.1 Database-Data Type

2.3 Data Sheet

2.3.1 Data Type

Data types in SQL Server 2005:

2.3.1.1 Exact numbers

Types of:

• Integer:

type of data	range	storage
int	-2^31 (-2,147,483,648) to 2^31-1 (2,147,483,647)	4 bytes
bigint	-2^63 (-9,223,372,036,854,775,808) to 2^63-1 (9,223,372,036,854,775,807)	8 bytes
smallint	-2^15 (-32,768) to 2^15-1(32,767)	2 bytes
tinyint	0 to 255	1 byte
bit	1，0（true，flase）	If there are 8 or less bit columns in the table, these columns share 1 byte for storage; if there are 9 to 16 bit columns, these columns are stored by 2 bytes, and so on.

• Numerical values ​​with fixed precision and decimal places:
  decimal and numeric (the two are equivalent)
  • Representation method: decimal[(p[,s])] and numeric[(p[,s])]
  • p (precision, precision): The total number of decimal digits that can be stored at most, including the digits to the left and right of the decimal point. The default precision is 18
  • s(scale, number of decimal places): The maximum number of decimal digits that can be stored to the right of the decimal point. The number of decimal places must be a value from 0 to p, that is, 0<=s<=p. The default number of decimal places is 0

Currency or monetary value:

2.3.1.2 Approximate numbers

Types of:

float	real

Role: used to indicate approximate values ​​(approximate values). Use when the size and number of digits are not clear, such as the result of dividing two trees, the result of square rooting, etc.

type of data	range	storage
float（n）	-1.79E+308 to -2.23E-308, 0 and 2.23E-308 to 1.79E+308	Depends on the value of n
real	-3.40+38 to -1.18E-38, 0 and 1.18E-38 to 3.40E+38	4 bytes

2.3.1.3 Date and time

Types of:

detetime	smalldatetime

Function: A data type that represents the date and time of a certain day.

type of data	range	Accuracy	storage
datetime	January 1, 1753 to December 31, 9999	3.33 ms	2*4 bytes
smalldatetime	January 1, 1900 to June 6, 2079	1 minute	2*2 bytes

2.3.1.4 String

Types of:

char	text
varchar

type of data	range	storage
char(n)	Fixed length, the length is n bytes. The value of n ranges from 1 to 8,000	n bytes
varchar(n/max)	Variable length, the value of n ranges from 1 to 8,000. max indicates that the maximum storage size is 2^31-1 bytes.	The actual length of the input data plus 2 bytes
text (will be cancelled in the future)	Variable-length character data, the maximum length is 2^31-1 characters.	Maximum 2^31-1 bytes

The choice of char and varchar:

• If the size of the column data items is consistent, char is used.
• If the size of the column data items differs considerably, use varchar.
• If the size of the column data items differs greatly, and the size may exceed 8,000 bytes, use varchar(max).

Type selection of long numeric data (such as student ID): generally select string

• Number type has length limitation;
• When using the numeric type, the leading zero of the data will be discarded;
• Data splicing is inconvenient;
• It is inconvenient to extract, replace, and modify certain digits in the data.

Choice of char(1), tinyint and bit:

2.3.1.5 Unicode strings

Unicode: Universal Chatacter Set (Universal Character Set)
Type:

nchar	ntext
nvarchar

type of data	range	storage
nchar(n)	Fixed-length Unicode character data of n characters. The value of n must be between 1 and 4,000.	2*n bytes
nvarchar(n/max)	Variable-length Unicode character data. The value of n is between 1 and 4,000. max indicates that the maximum storage size is 2^31-1 characters	Double the number of input characters + 2 bytes
ntext (will be cancelled in the future)	Variable-length Unicode data, the maximum length is 2^30-1 characters.	Enter twice the number of characters

The difference between char and nchar:

• Scope of application
• storage
• 1 length of nchar can represent 1 letter, number, symbol, Chinese character, 1 length of char can only represent 1 letter, number, symbol, 1 length of char can only represent 1 letter, number, symbol, Chinese character To use 2 lengths;
• When querying and comparing data, the space at the end of nchar is meaningful.

2.3.1.6 Binary string

Types of:

binary	image
varbinary

type of data	range	storage
binary(n)	Fixed-length binary data with a length of n bytes, where n is a value from 1 to 8,000.	n bytes
varbinary(n/max)	Variable-length binary data. n can take a value from 1 to 8,000. max indicates that the maximum storage size is 2^31-1 bytes.	The actual length of the input data plus 2 bytes
image (will be cancelled in the future)	Variable length binary data, from 0 to 2^31-1 bytes	Maximum 2^31-1 bytes

Storage method of multimedia data such as pictures:

• Store content in the table: use varbinary(max) to store.
  • Write data: convert the original data into binary data and store it in the table;
  • Read data: read out binary data and convert it into original data.
• Store the path in the table:
  • relative path
  • Absolute path

2.3.1.7 Other data types

Types of:

cursor	timestamp
sql_variant	uniqueidentifier
table	xml

type of data	effect
cuesor cursor	A data type of variable or stored procedure OUTPUT parameter
sql_variant variable type	Used to store the values ​​of various data types supported by SQL05 (excluding text, ntext, image, timestamp and sql_variant).
table 临时表	用于临时存储一组作为表值函数的结果集返回的行。

数据类型	作用	存储
timestamp 时间戳	给表加时间戳（每个数据库都有一个）计数器，当对数据库中 包含timestamp列的表执行插入或更新操作时，该计数器值就会增加。	8字节
uniqueidentifier	Globally Unique Identifier（GUID，全球唯一标识符）	16字节
xml	存储XML数据	最大2GB

2.3.1.8 用户自定义数据类型

⭐使用SQL Server Management Studio

• 方法：“对象资源管理器” → “数据库” → 选择数据库名称 → “可编程性” → “类型” → “用户定义数据类型” → 右键 → “新建用户定义的数据类型（N）…”。

2.3.1.9 常用数据类型（牢记）

数据类型		数据类新
精确数字	int decimal monkey	字符串	char varchar
近似数字	float real	Unicode 字符串	nchar nvarchar
日期时间	datetime	二进制字符串	varbinary

Sql Server 字段类型与C#数据类型对应表

Sql Server类型	C#类型	Sql Server类型	C#类型
image、binary、varbinary	byte[]	money、decimal、numeric	decimal
text、ntext、 char、varchar、 cchar、nvarchar	string	real	single
int	int	float	double
tnyint	byte	bit	bool
smallint	int16	bigint	int64
datetime、timestamp	DateTime