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nchar and nvarchar (Transact-SQL)

  • Artikkeli

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW)

Character data types that are either fixed-size, nchar, or variable-size, nvarchar. In SQL Server 2012 (11.x) and later versions, when a Supplementary Character (SC) enabled collation is used, these data types store the full range of Unicode character data and use the UTF-16 character encoding. If a non-SC collation is specified, then these data types store only the subset of character data supported by the UCS-2 character encoding.

Arguments

nchar [ ( n ) ]

Fixed-size string data. n defines the string size in byte-pairs, and must be a value from 1 through 4,000. The storage size is two times n bytes. For UCS-2 encoding, the storage size is two times n bytes and the number of characters that can be stored is also n. For UTF-16 encoding, the storage size is still two times n bytes, but the number of characters that can be stored might be smaller than n, because Supplementary Characters use two byte-pairs (also called surrogate pairs). The ISO synonyms for nchar are national char and national character.

nvarchar [ ( n | max ) ]

Variable-size string data. The value of n defines the string size in byte-pairs, and can be from 1 through 4,000. max indicates that the maximum storage size is 2^31-1 characters (2 GB). The storage size is two times n bytes + 2 bytes. For UCS-2 encoding, the storage size is two times n bytes + 2 bytes and the number of characters that can be stored is also n. For UTF-16 encoding, the storage size is still two times n bytes + 2 bytes. However, the number of characters that can be stored might be smaller than n, because Supplementary Characters use two byte-pairs (also called surrogate pairs). The ISO synonyms for nvarchar are national char varying and national character varying.

Remarks

A common misconception is to think that with nchar(n) and nvarchar(n), the n defines the number of characters. However, in nchar(n) and nvarchar(n), the n defines the string length in byte-pairs (0-4,000). n never defines numbers of characters that can be stored. This concept is similar to the definition of char and varchar.

The misconception happens because when using characters defined in the Unicode range 0 to 65,535, one character can be stored per each byte-pair. However, in higher Unicode ranges (65,536 to 1,114,111) one character might use two byte-pairs. For example, in a column defined as nchar(10), the Database Engine can store 10 characters that use one byte-pair (Unicode range 0 to 65,535), but fewer than 10 characters when using two byte-pairs (Unicode range 65,536 to 1,114,111). For more information about Unicode storage and character ranges, see Storage differences between UTF-8 and UTF-16.

When n isn't specified in a data definition or variable declaration statement, the default length is 1. When n isn't specified with the CAST function, the default length is 30.

If you use nchar or nvarchar, we recommend that you:

  • Use nchar when the sizes of the column data entries are consistent.
  • Use nvarchar when the sizes of the column data entries vary considerably.
  • Use nvarchar(max) when the sizes of the column data entries vary considerably, and the string length might exceed 4,000 byte-pairs.

sysname is a system-supplied user-defined data type that is functionally equivalent to nvarchar(128), except that it isn't nullable. sysname is used to reference database object names.

Objects that use nchar or nvarchar are assigned the default collation of the database unless a specific collation is assigned using the COLLATE clause.

SET ANSI_PADDING is always ON for nchar and nvarchar. SET ANSI_PADDING OFF doesn't apply to the nchar or nvarchar data types.

Prefix a Unicode character string constants with the letter N to signal UCS-2 or UTF-16 input, depending on whether an SC collation is used or not. Without the N prefix, the string is converted to the default code page of the database that might not recognize certain characters. In SQL Server 2019 (15.x) and later versions, when a UTF-8 enabled collation is used, the default code page is capable of storing the Unicode UTF-8 character set.

When you prefix a string constant with the letter N, the implicit conversion results in a UCS-2 or UTF-16 string if the constant to convert doesn't exceed the max length for the nvarchar string data type (4,000). Otherwise, the implicit conversion results in a large-value nvarchar(max).

Warning

Each non-null varchar(max) or nvarchar(max) column requires 24 bytes of additional fixed allocation, which counts against the 8,060-byte row limit during a sort operation. These additional bytes can create an implicit limit to the number of non-null varchar(max) or nvarchar(max) columns in a table. No special error is provided when the table is created (beyond the usual warning that the maximum row size exceeds the allowed maximum of 8,060 bytes) or at the time of data insertion. This large row size can cause errors (such as error 512) that users might not anticipate during some normal operations. Two examples of operations are a clustered index key update, or sorts of the full column set.

Convert character data

For information about converting character data, see char and varchar. For more information about converting between data types, see CAST and CONVERT.

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