When n is not specified in a data definition or variable declaration statement, the default length is 1. When n is not specified when using the CAST and CONVERT functions, the default length is 30.

Objects that use char or varchar are assigned the default collation of the database, unless a specific collation is assigned using the COLLATE clause. The collation controls the code page that is used to store the character data.

If you have sites that support multiple languages, consider using the Unicode nchar or nvarchar data types to minimize character conversion issues. If you use char or varchar, we recommend the following:

• Use char when the sizes of the column data entries are consistent.

• Use varchar when the sizes of the column data entries vary considerably.

• Use varchar(max) when the sizes of the column data entries vary considerably, and the size might exceed 8,000 bytes.

If SET ANSI_PADDING is OFF when either CREATE TABLE or ALTER TABLE is executed, a char column that is defined as NULL is handled as varchar.

When the collation code page uses double-byte characters, the storage size is still n bytes. Depending on the character string, the storage size of n bytes can be less than n characters.

When character expressions are converted to a character data type of a different size, values that are too long for the new data type are truncated. The uniqueidentifier type is considered a character type for the purposes of conversion from a character expression, and therefore is subject to the truncation rules for converting to a character type. See the Examples section that follows.

When a character expression is converted to a character expression of a different data type or size, such as from char(5) to varchar(5), or char(20) to char(15), the collation of the input value is assigned to the converted value. If a noncharacter expression is converted to a character data type, the default collation of the current database is assigned to the converted value. In either case, you can assign a specific collation by using the COLLATE clause.

Note
Code page translations are supported for char and varchar data types, but not for text data type. As with earlier versions of SQL Server, data loss during code page translations is not reported.

Character expressions that are being converted to an approximate numeric data type can include optional exponential notation (a lowercase e or uppercase E followed by an optional plus (+) or minus (-) sign and then a number).

Character expressions that are being converted to an exact numeric data type must consist of digits, a decimal point, and an optional plus (+) or minus (-). Leading blanks are ignored. Comma separators, such as the thousands separator in 123,456.00, are not allowed in the string.

Character expressions being converted to money or smallmoney data types can also include an optional decimal point and dollar sign ($). Comma separators, as in $123,456.00, are allowed.

A. Showing the default value of n when used in variable declaration.

DECLARE @myVariable AS varchar = 'abc';
DECLARE @myNextVariable AS char = 'abc';
--The following returns 1
SELECT DATALENGTH(@myVariable), DATALENGTH(@myNextVariable);
GO

B. Showing the default value of n when varchar is used with CAST and CONVERT.

DECLARE @myVariable AS varchar(40);
SET @myVariable = 'This string is longer than thirty characters';
SELECT CAST(@myVariable AS varchar);
SELECT DATALENGTH(CAST(@myVariable AS varchar)) AS 'VarcharDefaultLength';
SELECT CONVERT(char, @myVariable);
SELECT DATALENGTH(CONVERT(char, @myVariable)) AS 'VarcharDefaultLength';

C. Converting Data for Display Purposes

USE AdventureWorks2012;
GO
SELECT  BusinessEntityID, 
   SalesYTD, 
   CONVERT (varchar(12),SalesYTD,1) AS MoneyDisplayStyle1, 
   GETDATE() AS CurrentDate, 
   CONVERT(varchar(12), GETDATE(), 3) AS DateDisplayStyle3
FROM Sales.SalesPerson
WHERE CAST(SalesYTD AS varchar(20) ) LIKE '1%';

BusinessEntityID SalesYTD              DisplayFormat CurrentDate             DisplayDateFormat
---------------- --------------------- ------------- ----------------------- -----------------
278              1453719.4653          1,453,719.47  2011-05-07 14:29:01.193 07/05/11
280              1352577.1325          1,352,577.13  2011-05-07 14:29:01.193 07/05/11
283              1573012.9383          1,573,012.94  2011-05-07 14:29:01.193 07/05/11
284              1576562.1966          1,576,562.20  2011-05-07 14:29:01.193 07/05/11
285              172524.4512           172,524.45    2011-05-07 14:29:01.193 07/05/11
286              1421810.9242          1,421,810.92  2011-05-07 14:29:01.193 07/05/11
288              1827066.7118          1,827,066.71  2011-05-07 14:29:01.193 07/05/11

D. Converting Uniqueidentifer Data

DECLARE @myid uniqueidentifier = NEWID();
SELECT CONVERT(char(255), @myid) AS 'char';

DECLARE @ID nvarchar(max) = N'0E984725-C51C-4BF4-9960-E1C80E27ABA0wrong';
SELECT @ID, CONVERT(uniqueidentifier, @ID) AS TruncatedValue;

String                                       TruncatedValue
-------------------------------------------- ------------------------------------
0E984725-C51C-4BF4-9960-E1C80E27ABA0wrong    0E984725-C51C-4BF4-9960-E1C80E27ABA0

(1 row(s) affected)

Reference

Concepts

Other Resources