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SQL Server CAST and CONVERT (from Microsoft msdn) 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.

data_type

Is the target system-supplied data type, including bigint and sql_variant. User-defined data types cannot be used.

length

Is an optional parameter of nchar, nvarchar, char, varchar, binary, or varbinary data types.

style

Is the style of date format used to convert 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).

SQL Server supports the date format in Arabic style, using Kuwaiti algorithm.

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).

*    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.
*** Designed for XML use. For conversion from datetime or smalldatetime to character data, the output format is as described in the table. For conversion from float, money, or smallmoney to character data, the output is equivalent to style 2. For conversion from real to character data, the output is equivalent to style 1.
****Hijri is a calendar system with several variations, of which Microsoft® SQL Server™ 2000 uses the Kuwaiti algorithm.

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.

In the following table, the column on the left represents the style value for money or smallmoney conversion to character data.

Return Types

Returns the same value as data type 0.

Remarks

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, including bigint and sql_variant.

Note  Because Unicode data always uses 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 is 8000. If you attempt an incorrect conversion (for example, if you convert a character expression that includes letters to an int), SQL Server generates an error message.

When the output of CAST or CONVERT is a character string, and the input is a character string, the output has the same collation and collation label as the input. If the input is not a character string, the output has the default collation of the database, and a collation label of coercible-default.

To assign a different collation to the output, apply the COLLATE clause to the result expression of the CAST or CONVERT function. For example:

SELECT CAST('abc' AS varchar(5)) COLLATE French_CS_AS

There is no implicit conversion on assignment from the sql_variant data type but there is implicit conversion to sql_variant.

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.

* 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 = 193.57
SELECT CAST(CAST(@myval AS varbinary(20)) AS decimal(10,5))
-- Or, using CONVERT
SELECT 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.

This example shows a resulting expression too small to display.

USE pubs
SELECT SUBSTRING(title, 1, 25) AS Title, CAST(ytd_sales AS char(2))
FROM titles
WHERE 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 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.

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(10.3496847 AS money) is $10.3497.

SQL Server returns an error message when non-numeric char, nchar, varchar, or nvarchar data is converted to int, float, numeric, or decimal. SQL Server also returns an error when an empty string (" ") is converted to numeric or decimal.

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 from 0 through 9 and from A through F (or from a 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 pubs
GO
SELECT SUBSTRING(title, 1, 30) AS Title, ytd_sales
FROM titles
WHERE CAST(ytd_sales AS char(20)) LIKE '3%'
GO

-- Use CONVERT.
USE pubs
GO
SELECT SUBSTRING(title, 1, 30) AS Title, ytd_sales
FROM titles
WHERE 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) by the individual book price (price). This result is converted to an int data type after being rounded to the nearest whole number.

USE pubs
GO
SELECT CAST(ROUND(ytd_sales/price, 0) AS int) AS 'Copies'
FROM titles
GO

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 pubs
GO
SELECT 'The price is ' + CAST(price AS varchar(12))
FROM titles
WHERE price > 10.00
GO

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 pubs
GO
SELECT CAST(title AS char(50)), ytd_sales
FROM titles
WHERE type = 'trad_cook'
GO

Here is the result set:

                                                       ytd_sales
--------------------------------------------------     ---------
Onions, Leeks, and Garlic: Cooking Secrets of the      375
Fifty Years in Buckingham Palace Kitchens              15096
Sushi, 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 pubs
GO
SELECT title, ytd_sales
FROM titles
WHERE 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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