Типът данни определя за дадена колона каква стойност може да притежава: цели числа, данни за знаци, парични данни, данни за дата и час, двоични низове и т.н.
SQL типове данни
Всяка колона в таблица на базата данни трябва да има име и тип данни.
Един разработчик на SQL трябва да реши какъв тип данни ще се съхраняват във всяка колона, когато създава таблицата.
Типът данни е насока за SQL, за да се разбере какъв тип данни се очаква да се въвеждат във всяка колона и също така идентифицира как SQL ще взаимодейства със запаметените данни.
Забележка: Типовете данни може да имат различни имена в различни бази данни. И дори ако името е същото, размерът и другите подробности може да са различни! Винаги проверявайте документацията!
MySQL типове данни
В MySQL има три основни вида данни: текст, числа и дати.
Типове текстови данни:
| Тип | Описание |
|---|---|
| CHAR(size) | Holds a fixed length string (can contain letters, numbers, and special characters). The fixed size is specified in parenthesis. Can store up to 255 characters |
| VARCHAR(size) | Holds a variable length string (can contain letters, numbers, and special characters). The maximum size is specified in parenthesis. Can store up to 255 characters. Note: If you put a greater value than 255 it will be converted to a TEXT type |
| TINYTEXT | Съдържа стринг с максимална дължина от 255 знака/символа |
| TEXT | Съдържа стринг с максимална дължина от 65,535 знака/символа |
| BLOB | For BLOBs (Binary Large OBjects). Holds up to 65,535 bytes of data |
| MEDIUMTEXT | Holds a string with a maximum length of 16,777,215 characters |
| MEDIUMBLOB | For BLOBs (Binary Large OBjects). Holds up to 16,777,215 bytes of data |
| LONGTEXT | Holds a string with a maximum length of 4,294,967,295 characters |
| LONGBLOB | For BLOBs (Binary Large OBjects). Holds up to 4,294,967,295 bytes of data |
| ENUM(x,y,z,etc.) | Let you enter a list of possible values. You can list up to 65535 values in an ENUM list. If a value is inserted that is not in the list, a blank value will be inserted.
Note: The values are sorted in the order you enter them. You enter the possible values in this format: ENUM(‘X’,’Y’,’Z’) |
| SET | Similar to ENUM except that SET may contain up to 64 list items and can store more than one choice |
Числови типове данни:
| Тип | Описание |
|---|---|
| TINYINT(size) | -128 to 127 normal. 0 to 255 UNSIGNED*. The maximum number of digits may be specified in parenthesis |
| SMALLINT(size) | -32768 to 32767 normal. 0 to 65535 UNSIGNED*. The maximum number of digits may be specified in parenthesis |
| MEDIUMINT(size) | -8388608 to 8388607 normal. 0 to 16777215 UNSIGNED*. The maximum number of digits may be specified in parenthesis |
| INT(size) | -2147483648 to 2147483647 normal. 0 to 4294967295 UNSIGNED*. The maximum number of digits may be specified in parenthesis |
| BIGINT(size) | -9223372036854775808 to 9223372036854775807 normal. 0 to 18446744073709551615 UNSIGNED*. The maximum number of digits may be specified in parenthesis |
| FLOAT(size,d) | A small number with a floating decimal point. The maximum number of digits may be specified in the size parameter. The maximum number of digits to the right of the decimal point is specified in the d parameter |
| DOUBLE(size,d) | A large number with a floating decimal point. The maximum number of digits may be specified in the size parameter. The maximum number of digits to the right of the decimal point is specified in the d parameter |
| DECIMAL(size,d) | A DOUBLE stored as a string , allowing for a fixed decimal point. The maximum number of digits may be specified in the size parameter. The maximum number of digits to the right of the decimal point is specified in the d parameter |
* Цифровите типове имат допълнителна опция, наречена UNSIGNED. Обикновено цяло число преминава от отрицателна към положителна стойност. Добавянето на атрибута UNSIGNED ще премести този диапазон, така че да започва от нула вместо отрицателен номер.
Типове данни за дата:
| Тип | Описание |
|---|---|
| DATE() | A date. Format: YYYY-MM-DD Note: The supported range is from ‘1000-01-01’ to ‘9999-12-31’ |
| DATETIME() | *A date and time combination. Format: YYYY-MM-DD HH:MI:SS Note: The supported range is from ‘1000-01-01 00:00:00’ to ‘9999-12-31 23:59:59’ |
| TIMESTAMP() | *A timestamp. TIMESTAMP values are stored as the number of seconds since the Unix epoch (‘1970-01-01 00:00:00’ UTC). Format: YYYY-MM-DD HH:MI:SS Note: The supported range is from ‘1970-01-01 00:00:01’ UTC to ‘2038-01-09 03:14:07’ UTC |
| TIME() | A time. Format: HH:MI:SS Note: The supported range is from ‘-838:59:59’ to ‘838:59:59’ |
| YEAR() | A year in two-digit or four-digit format.
Note: Values allowed in four-digit format: 1901 to 2155. Values allowed in two-digit format: 70 to 69, representing years from 1970 to 2069 |
* Дори ако DATETIME и TIMESTAMP върнат същия формат, те работят съвсем различно. При заявка INSERT или UPDATE TIMESTAMP автоматично се настройва на текущата дата и час. TIMESTAMP също приема различни формати, като YYYYMMDDHHMISS, YYMMDDHHMISS, YYYYMMDD или YYMMDD.
Типове данни на SQL Server
Типове данни за низове:
| Data type | Описание | Max size | Storage |
|---|---|---|---|
| char(n) | Fixed width character string | 8,000 characters | Defined width |
| varchar(n) | Variable width character string | 8,000 characters | 2 bytes + number of chars |
| varchar(max) | Variable width character string | 1,073,741,824 characters | 2 bytes + number of chars |
| text | Variable width character string | 2GB of text data | 4 bytes + number of chars |
| nchar | Fixed width Unicode string | 4,000 characters | Defined width x 2 |
| nvarchar | Variable width Unicode string | 4,000 characters | |
| nvarchar(max) | Variable width Unicode string | 536,870,912 characters | |
| ntext | Variable width Unicode string | 2GB of text data | |
| binary(n) | Fixed width binary string | 8,000 bytes | |
| varbinary | Variable width binary string | 8,000 bytes | |
| varbinary(max) | Variable width binary string | 2GB | |
| image | Variable width binary string | 2GB |
Типове типове данни:
| Data type | Описание | Storage |
|---|---|---|
| bit | Integer that can be 0, 1, or NULL | |
| tinyint | Allows whole numbers from 0 to 255 | 1 byte |
| smallint | Allows whole numbers between -32,768 and 32,767 | 2 bytes |
| int | Allows whole numbers between -2,147,483,648 and 2,147,483,647 | 4 bytes |
| bigint | Allows whole numbers between -9,223,372,036,854,775,808 and 9,223,372,036,854,775,807 | 8 bytes |
| decimal(p,s) | Fixed precision and scale numbers.
Allows numbers from -10^38 +1 to 10^38 –1. The p parameter indicates the maximum total number of digits that can be stored (both to the left and to the right of the decimal point). p must be a value from 1 to 38. Default is 18. The s parameter indicates the maximum number of digits stored to the right of the decimal point. s must be a value from 0 to p. Default value is 0 |
5-17 bytes |
| numeric(p,s) | Fixed precision and scale numbers.
Allows numbers from -10^38 +1 to 10^38 –1. The p parameter indicates the maximum total number of digits that can be stored (both to the left and to the right of the decimal point). p must be a value from 1 to 38. Default is 18. The s parameter indicates the maximum number of digits stored to the right of the decimal point. s must be a value from 0 to p. Default value is 0 |
5-17 bytes |
| smallmoney | Monetary data from -214,748.3648 to 214,748.3647 | 4 bytes |
| money | Monetary data from -922,337,203,685,477.5808 to 922,337,203,685,477.5807 | 8 bytes |
| float(n) | Floating precision number data from -1.79E + 308 to 1.79E + 308. The n parameter indicates whether the field should hold 4 or 8 bytes. float(24) holds a 4-byte field and float(53) holds an 8-byte field. Default value of n is 53. |
4 or 8 bytes |
| real | Floating precision number data from -3.40E + 38 to 3.40E + 38 | 4 bytes |
Типове данни за дата:
| Data type | Описание | Storage |
|---|---|---|
| datetime | From January 1, 1753 to December 31, 9999 with an accuracy of 3.33 milliseconds | 8 bytes |
| datetime2 | From January 1, 0001 to December 31, 9999 with an accuracy of 100 nanoseconds | 6-8 bytes |
| smalldatetime | From January 1, 1900 to June 6, 2079 with an accuracy of 1 minute | 4 bytes |
| date | Store a date only. From January 1, 0001 to December 31, 9999 | 3 bytes |
| time | Store a time only to an accuracy of 100 nanoseconds | 3-5 bytes |
| datetimeoffset | The same as datetime2 with the addition of a time zone offset | 8-10 bytes |
| timestamp | Stores a unique number that gets updated every time a row gets created or modified. The timestamp value is based upon an internal clock and does not correspond to real time. Each table may have only one timestamp variable |
Други типове данни:
| Data type | Описание |
|---|---|
| sql_variant | Stores up to 8,000 bytes of data of various data types, except text, ntext, and timestamp |
| uniqueidentifier | Stores a globally unique identifier (GUID) |
| xml | Stores XML formatted data. Maximum 2GB |
| cursor | Stores a reference to a cursor used for database operations |
| table | Stores a result-set for later processing |
Типове данни на Microsoft Access
| Data type | Описание | Storage |
|---|---|---|
| Text | Use for text or combinations of text and numbers. 255 characters maximum | |
| Memo | Memo is used for larger amounts of text. Stores up to 65,536 characters. Note: You cannot sort a memo field. However, they are searchable | |
| Byte | Allows whole numbers from 0 to 255 | 1 byte |
| Integer | Allows whole numbers between -32,768 and 32,767 | 2 bytes |
| Long | Allows whole numbers between -2,147,483,648 and 2,147,483,647 | 4 bytes |
| Single | Single precision floating-point. Will handle most decimals | 4 bytes |
| Double | Double precision floating-point. Will handle most decimals | 8 bytes |
| Currency | Use for currency. Holds up to 15 digits of whole dollars, plus 4 decimal places. Tip: You can choose which country’s currency to use | 8 bytes |
| AutoNumber | AutoNumber fields automatically give each record its own number, usually starting at 1 | 4 bytes |
| Date/Time | Use for dates and times | 8 bytes |
| Yes/No | A logical field can be displayed as Yes/No, True/False, or On/Off. In code, use the constants True and False (equivalent to -1 and 0). Note: Null values are not allowed in Yes/No fields | 1 bit |
| Ole Object | Can store pictures, audio, video, or other BLOBs (Binary Large OBjects) | up to 1GB |
| Hyperlink | Contain links to other files, including web pages | |
| Lookup Wizard | Let you type a list of options, which can then be chosen from a drop-down list | 4 bytes |