CAST and CONVERT (T-SQL) Explicitly converts an expression of one data type to another. CAST and CONVERT provide similar functionality.Syntax Using CAST:CAST(expression AS data_type)Using CONVERT:CONVERT (data_type[(length)], expression [, style])Arguments expression Is any valid Microsoft® SQL Server™ expression. For more information, see Expressions. data_type Is the target system-supplied data type. User-defined data types cannot be used. For more information about available data types, see Data Types. length Is an optional parameter of nchar, nvarchar, char, varchar, binary, or varbinary data types. style Is the style of date format you want when converting datetime or smalldatetime data to character data (nchar, nvarchar, char, varchar, nchar, or nvarchar data types), or the string format when converting float, real, money, or smallmoney data to character data (nchar, nvarchar, char, varchar, nchar, or nvarchar data types). In the table, the two columns on the left represent the style values for datetime or smalldatetime conversion to character data. Add 100 to a style value to get a four-place year that includes the century (yyyy). Without century (yy) With century (yyyy) Standard Input/Output** - 0 or 100 (*) Default mon dd yyyy hh:miAM (or PM) 1 101 USA mm/dd/yy 2 102 ANSI yy.mm.dd 3 103 British/French dd/mm/yy 4 104 German dd.mm.yy 5 105 Italian dd-mm-yy 6 106 - dd mon yy 7 107 - mon dd, yy 8 108 - hh:mm:ss - 9 or 109 (*) Default + milliseconds mon dd yyyy hh:mi:ss:mmmAM (or PM) 10 110 USA mm-dd-yy 11 111 JAPAN yy/mm/dd 12 112 ISO yymmdd - 13 or 113 (*) Europe default + milliseconds dd mon yyyy hh:mm:ss:mmm(24h) 14 114 - hh:mi:ss:mmm(24h) - 20 or 120 (*) ODBC canonical yyyy-mm-dd hh:mi:ss(24h) - 21 or 121 (*) ODBC canonical (with milliseconds) yyyy-mm-dd hh:mi:ss.mmm(24h) * The default values (style 0 or 100, 9 or 109, 13 or 113, 20 or 120, and 21 or 121) always return the century (yyyy). ** Input when converting to datetime; Output when converting to character data.
--------------------------------------------------------------------------------Important By default, SQL Server interprets two-digit years based on a cutoff year of 2049. That is, the two-digit year 49 is interpreted as 2049 and the two-digit year 50 is interpreted as 1950. Many client applications, such as those based on OLE Automation objects, use a cutoff year of 2030. SQL Server provides a configuration option (two digit year cutoff) that changes the cutoff year used by SQL Server and allows the consistent treatment of dates. The safest course, however, is to specify four-digit years. --------------------------------------------------------------------------------When you convert to character data from smalldatetime, the styles that include seconds or milliseconds show zeros in these positions. You can truncate unwanted date parts when converting from datetime or smalldatetime values by using an appropriate char or varchar data type length. This table shows the style values for float or real conversion to character data. Value Output 0 (the default) 6 digits maximum. Use in scientific notation, when appropriate. 1 Always 8 digits. Always use in scientific notation. 2 Always 16 digits. Always use in scientific notation. In the following table, the column on the left represents the style value for money or smallmoney conversion to character data. Value Output 0 (the default) No commas every three digits to the left of the decimal point, and two digits to the right of the decimal point; for example, 4235.98. 1 Commas every three digits to the left of the decimal point, and two digits to the right of the decimal point; for example, 3,510.92. 2 No commas every three digits to the left of the decimal point, and four digits to the right of the decimal point; for example, 4235.9819. Return Types Returns the same value as data type 0.Res Implicit conversions are those conversions that occur without specifying either the CAST or CONVERT function. Explicit conversions are those conversions that require the CAST (CONVERT) function to be specified. This chart shows all explicit and implicit data type conversions allowed for SQL Server system-supplied data types. --------------------------------------------------------------------------------Note Because Unicode data always use an even number of bytes, use caution when converting binary or varbinary to or from Unicode supported data types. For example, this conversion does not return a hexadecimal value of 41, but of 4100: SELECT CAST(CAST(0x41 AS nvarchar) AS varbinary)--------------------------------------------------------------------------------Automatic data type conversion is not supported for the text and image data types. You can explicitly convert text data to character data, and image data to binary or varbinary, but the maximum length you can specify is 8000. If you attempt a conversion that is not possible (for example, if you convert a character expression that includes letters to an int), SQL Server generates an error message.When converting character or binary expressions (char, nchar, nvarchar, varchar, binary, or varbinary) to an expression of a different data type, data can be truncated, only partially displayed, or an error is returned because the result is too short to display. Conversions to char, varchar, nchar, nvarchar, binary, and varbinary are truncated, except for the conversions shown in this table.From data type To data type Result int, smallint, or tinyint char * varchar * nchar E nvarchar E money, smallmoney, numeric, decimal, float, or real char E varchar E nchar E nvarchar E * Result length too short to display. E Error returned because result length is too short to display.
Microsoft SQL Server guarantees that only roundtrip conversions, conversions that convert a data type from its original data type and back again, will yield the same values from release to release. This example shows such a roundtrip conversion:DECLARE @myval decimal (5, 2)SET @myval = 193.57SELECT CAST(CAST(@myval AS varbinary(20)) AS decimal(10,5))-- Or, using CONVERTSELECT CONVERT(decimal(10,5), CONVERT(varbinary(20), @myval)) Do not attempt to construct, for example, binary values and convert them to a data type of the numeric data type category. SQL Server does not guarantee that the result of a decimal or numeric data type conversion to binary will be the same between releases of SQL Server.When data types are converted with a different number of decimal places, the value is truncated to the most precise digit. For example, the result of SELECT CAST(10.6496 AS int) is 10.This example shows a resulting expression that is too small to display.USE pubsSELECT SUBSTRING(title, 1, 25) AS Title, CAST(ytd_sales AS char(2))FROM titlesWHERE type = 'trad_cook' Here is the result set:Title ------------------------- -- Onions, Leeks, and Garlic * Fifty Years in Buckingham * Sushi, Anyone? * (3 row(s) affected) When data types in which the target data type has fewer decimal points than the source data type are converted, the value is rounded. For example, the result of CAST(10.3496 AS money) is $10.35.SQL Server returns an error message when char, nchar, varchar, or nvarchar data is converted to int, float, numeric, or decimal; or when an empty string (“ “) is converted to int or float.Using Binary String Data When binary or varbinary data is converted to character data and an odd number of values is specified following the x, SQL Server adds a 0 (zero) after the x to make an even number of values.Binary data consists of the characters 0 through 9 and A through F (or through f), in groups of two characters each. Binary strings must be preceded by 0x. For example, to input FF, type 0xFF. The maximum value is a binary value of 8000 bytes, each of which is FF. The binary data types are not for hexadecimal data, but rather for bit patterns. Conversions and calculations of hexadecimal numbers stored as binary data can be unreliable.When specifying the length of a binary data type, every two characters count as one. A length of 10 signifies that 10 two-character groupings will be entered.Empty binary strings, represented by 0x, can be stored as binary data.Examples A. Use both CAST and CONVERT Each example retrieves the titles for those books that have a 3 in the first digit of year-to-date sales, and converts their ytd_sales to char(20).-- Use CAST.USE pubsGOSELECT SUBSTRING(title, 1, 30) AS Title, ytd_salesFROM titlesWHERE CAST(ytd_sales AS char(20)) LIKE '3%'GO -- Use CONVERT.USE pubsGOSELECT SUBSTRING(title, 1, 30) AS Title, ytd_salesFROM titlesWHERE CONVERT(char(20), ytd_sales) LIKE '3%'GO Here is the result set for either query:Title ytd_sales ------------------------------ ----------- Cooking with Computers: Surrep 3876 Computer Phobic AND Non-Phobic 375 Emotional Security: A New Algo 3336 Onions, Leeks, and Garlic: Coo 375 (4 row(s) affected) B. Use CAST with arithmetic operators This example calculates a single column computation (Copies) by dividing the total year-to-date sales (ytd_sales) divided by the individual book price (price). This result is converted to an int data type after being rounded to the nearest whole number. USE pubsGOSELECT CAST(ROUND(ytd_sales/price, 0) AS int) AS 'Copies'FROM titlesGO Here is the result set:Copies ----------- 205 324 6262 205 102 7440 NULL 383 205 NULL 17 187 16 204 418 18 1263 273 (18 row(s) affected) C. Use CAST to concatenate This example concatenates noncharacter, nonbinary expressions using the CAST data type conversion function.USE pubsGOSELECT 'The price is ' + CAST(price AS varchar(12))FROM titlesWHERE price > 10.00GO Here is the result set:------------------------- The price is 19.99 The price is 11.95 The price is 19.99 The price is 19.99 The price is 22.95 The price is 20.00 The price is 21.59 The price is 10.95 The price is 19.99 The price is 20.95 The price is 11.95 The price is 14.99 (12 row(s) affected) D. Use CAST for more readable text This example uses CAST in the select list to convert the title column to a char(50) column so the results are more readable.USE pubsGOSELECT CAST(title AS char(50)), ytd_sales FROM titlesWHERE type = 'trad_cook'GO Here is the result set: ytd_sales-------------------------------------------------- ---------Onions, Leeks, and Garlic: Cooking Secrets of the 375Fifty Years in Buckingham Palace Kitchens 15096Sushi, Anyone? 4095 (3 row(s) affected) E. Use CAST with LIKE clause This example converts an int column (the ytd_sales column) to a char(20) column so that it can be used with the LIKE clause.USE pubsGOSELECT title, ytd_salesFROM titlesWHERE CAST(ytd_sales AS char(20)) LIKE '15%' AND type = 'trad_cook'GO Here is the result set:title ytd_sales ------------------------------------------------------------ ----------- Fifty Years in Buckingham Palace Kitchens 15096 (1 row(s) affected)
是的convert 有很多参数的
Explicitly converts an expression of one data type to another. CAST and CONVERT provide similar functionality.Syntax
Using CAST:CAST(expression AS data_type)Using CONVERT:CONVERT (data_type[(length)], expression [, style])Arguments
expression
Is any valid Microsoft® SQL Server™ expression. For more information, see Expressions.
data_type
Is the target system-supplied data type. User-defined data types cannot be used. For more information about available data types, see Data Types.
length
Is an optional parameter of nchar, nvarchar, char, varchar, binary, or varbinary data types.
style
Is the style of date format you want when converting datetime or smalldatetime data to character data (nchar, nvarchar, char, varchar, nchar, or nvarchar data types), or the string format when converting float, real, money, or smallmoney data to character data (nchar, nvarchar, char, varchar, nchar, or nvarchar data types).
In the table, the two columns on the left represent the style values for datetime or smalldatetime conversion to character data. Add 100 to a style value to get a four-place year that includes the century (yyyy). Without
century
(yy) With
century
(yyyy) Standard Input/Output**
- 0 or 100 (*) Default mon dd yyyy hh:miAM (or PM)
1 101 USA mm/dd/yy
2 102 ANSI yy.mm.dd
3 103 British/French dd/mm/yy
4 104 German dd.mm.yy
5 105 Italian dd-mm-yy
6 106 - dd mon yy
7 107 - mon dd, yy
8 108 - hh:mm:ss
- 9 or 109 (*) Default + milliseconds mon dd yyyy hh:mi:ss:mmmAM (or PM)
10 110 USA mm-dd-yy
11 111 JAPAN yy/mm/dd
12 112 ISO yymmdd
- 13 or 113 (*) Europe default + milliseconds dd mon yyyy hh:mm:ss:mmm(24h)
14 114 - hh:mi:ss:mmm(24h)
- 20 or 120 (*) ODBC canonical yyyy-mm-dd hh:mi:ss(24h)
- 21 or 121 (*) ODBC canonical (with milliseconds) yyyy-mm-dd hh:mi:ss.mmm(24h)
* The default values (style 0 or 100, 9 or 109, 13 or 113, 20 or 120, and 21 or 121) always return the century (yyyy).
** Input when converting to datetime; Output when converting to character data.
--------------------------------------------------------------------------------Important By default, SQL Server interprets two-digit years based on a cutoff year of 2049. That is, the two-digit year 49 is interpreted as 2049 and the two-digit year 50 is interpreted as 1950. Many client applications, such as those based on OLE Automation objects, use a cutoff year of 2030. SQL Server provides a configuration option (two digit year cutoff) that changes the cutoff year used by SQL Server and allows the consistent treatment of dates. The safest course, however, is to specify four-digit years.
--------------------------------------------------------------------------------When you convert to character data from smalldatetime, the styles that include seconds or milliseconds show zeros in these positions. You can truncate unwanted date parts when converting from datetime or smalldatetime values by using an appropriate char or varchar data type length. This table shows the style values for float or real conversion to character data. Value Output
0 (the default) 6 digits maximum. Use in scientific notation, when appropriate.
1 Always 8 digits. Always use in scientific notation.
2 Always 16 digits. Always use in scientific notation.
In the following table, the column on the left represents the style value for money or smallmoney conversion to character data. Value Output
0 (the default) No commas every three digits to the left of the decimal point, and two digits to the right of the decimal point; for example, 4235.98.
1 Commas every three digits to the left of the decimal point, and two digits to the right of the decimal point; for example, 3,510.92.
2 No commas every three digits to the left of the decimal point, and four digits to the right of the decimal point; for example, 4235.9819.
Return Types
Returns the same value as data type 0.Res
Implicit conversions are those conversions that occur without specifying either the CAST or CONVERT function. Explicit conversions are those conversions that require the CAST (CONVERT) function to be specified. This chart shows all explicit and implicit data type conversions allowed for SQL Server system-supplied data types.
--------------------------------------------------------------------------------Note Because Unicode data always use an even number of bytes, use caution when converting binary or varbinary to or from Unicode supported data types. For example, this conversion does not return a hexadecimal value of 41, but of 4100:
SELECT CAST(CAST(0x41 AS nvarchar) AS varbinary)--------------------------------------------------------------------------------Automatic data type conversion is not supported for the text and image data types. You can explicitly convert text data to character data, and image data to binary or varbinary, but the maximum length you can specify is 8000. If you attempt a conversion that is not possible (for example, if you convert a character expression that includes letters to an int), SQL Server generates an error message.When converting character or binary expressions (char, nchar, nvarchar, varchar, binary, or varbinary) to an expression of a different data type, data can be truncated, only partially displayed, or an error is returned because the result is too short to display. Conversions to char, varchar, nchar, nvarchar, binary, and varbinary are truncated, except for the conversions shown in this table.From data type To data type Result
int, smallint, or tinyint char *
varchar *
nchar E
nvarchar E
money, smallmoney, numeric, decimal, float, or real char E
varchar E
nchar E
nvarchar E
* Result length too short to display.
E Error returned because result length is too short to display.
Microsoft SQL Server guarantees that only roundtrip conversions, conversions that convert a data type from its original data type and back again, will yield the same values from release to release. This example shows such a roundtrip conversion:DECLARE @myval decimal (5, 2)SET @myval = 193.57SELECT CAST(CAST(@myval AS varbinary(20)) AS decimal(10,5))-- Or, using CONVERTSELECT CONVERT(decimal(10,5), CONVERT(varbinary(20), @myval))
Do not attempt to construct, for example, binary values and convert them to a data type of the numeric data type category. SQL Server does not guarantee that the result of a decimal or numeric data type conversion to binary will be the same between releases of SQL Server.When data types are converted with a different number of decimal places, the value is truncated to the most precise digit. For example, the result of SELECT CAST(10.6496 AS int) is 10.This example shows a resulting expression that is too small to display.USE pubsSELECT SUBSTRING(title, 1, 25) AS Title, CAST(ytd_sales AS char(2))FROM titlesWHERE type = 'trad_cook' Here is the result set:Title ------------------------- -- Onions, Leeks, and Garlic * Fifty Years in Buckingham * Sushi, Anyone? * (3 row(s) affected) When data types in which the target data type has fewer decimal points than the source data type are converted, the value is rounded. For example, the result of CAST(10.3496 AS money) is $10.35.SQL Server returns an error message when char, nchar, varchar, or nvarchar data is converted to int, float, numeric, or decimal; or when an empty string (“ “) is converted to int or float.Using Binary String Data
When binary or varbinary data is converted to character data and an odd number of values is specified following the x, SQL Server adds a 0 (zero) after the x to make an even number of values.Binary data consists of the characters 0 through 9 and A through F (or through f), in groups of two characters each. Binary strings must be preceded by 0x. For example, to input FF, type 0xFF. The maximum value is a binary value of 8000 bytes, each of which is FF. The binary data types are not for hexadecimal data, but rather for bit patterns. Conversions and calculations of hexadecimal numbers stored as binary data can be unreliable.When specifying the length of a binary data type, every two characters count as one. A length of 10 signifies that 10 two-character groupings will be entered.Empty binary strings, represented by 0x, can be stored as binary data.Examples
A. Use both CAST and CONVERT
Each example retrieves the titles for those books that have a 3 in the first digit of year-to-date sales, and converts their ytd_sales to char(20).-- Use CAST.USE pubsGOSELECT SUBSTRING(title, 1, 30) AS Title, ytd_salesFROM titlesWHERE CAST(ytd_sales AS char(20)) LIKE '3%'GO -- Use CONVERT.USE pubsGOSELECT SUBSTRING(title, 1, 30) AS Title, ytd_salesFROM titlesWHERE CONVERT(char(20), ytd_sales) LIKE '3%'GO Here is the result set for either query:Title ytd_sales ------------------------------ ----------- Cooking with Computers: Surrep 3876 Computer Phobic AND Non-Phobic 375 Emotional Security: A New Algo 3336 Onions, Leeks, and Garlic: Coo 375 (4 row(s) affected) B. Use CAST with arithmetic operators
This example calculates a single column computation (Copies) by dividing the total year-to-date sales (ytd_sales) divided by the individual book price (price). This result is converted to an int data type after being rounded to the nearest whole number. USE pubsGOSELECT CAST(ROUND(ytd_sales/price, 0) AS int) AS 'Copies'FROM titlesGO Here is the result set:Copies ----------- 205 324 6262 205 102 7440 NULL 383 205 NULL 17 187 16 204 418 18 1263 273 (18 row(s) affected) C. Use CAST to concatenate
This example concatenates noncharacter, nonbinary expressions using the CAST data type conversion function.USE pubsGOSELECT 'The price is ' + CAST(price AS varchar(12))FROM titlesWHERE price > 10.00GO Here is the result set:------------------------- The price is 19.99 The price is 11.95 The price is 19.99 The price is 19.99 The price is 22.95 The price is 20.00 The price is 21.59 The price is 10.95 The price is 19.99 The price is 20.95 The price is 11.95 The price is 14.99 (12 row(s) affected) D. Use CAST for more readable text
This example uses CAST in the select list to convert the title column to a char(50) column so the results are more readable.USE pubsGOSELECT CAST(title AS char(50)), ytd_sales FROM titlesWHERE type = 'trad_cook'GO Here is the result set: ytd_sales-------------------------------------------------- ---------Onions, Leeks, and Garlic: Cooking Secrets of the 375Fifty Years in Buckingham Palace Kitchens 15096Sushi, Anyone? 4095 (3 row(s) affected) E. Use CAST with LIKE clause
This example converts an int column (the ytd_sales column) to a char(20) column so that it can be used with the LIKE clause.USE pubsGOSELECT title, ytd_salesFROM titlesWHERE CAST(ytd_sales AS char(20)) LIKE '15%' AND type = 'trad_cook'GO Here is the result set:title ytd_sales ------------------------------------------------------------ ----------- Fifty Years in Buckingham Palace Kitchens 15096 (1 row(s) affected)
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下几本Sql Server的书看看吧
convert