An identifier that complies with all the rules for the format of identifiers can be used with or without delimiters. An identifier that does not comply with the rules for the format of regular identifiers must always be delimited.

Delimited identifiers are used in these situations

:

• When reserved words are used for object names or portions of object names.

  It is recommended that reserved keywords not be used as object names. Databases upgraded from earlier versions of Microsoft® SQL Server™ may contain identifiers that include words not reserved in the earlier version, but are reserved words for SQL Server 2000. You can refer to the object using delimited identifiers until the name can be changed.

• When using characters not listed as qualified identifiers.

  SQL Server allows any character in the current code page to be used in a delimited identifier; however, indiscriminate use of special characters in an object name may make SQL statements and scripts difficult to read and maintain.

Types of delimiters used in Transact-SQL:

Note  Delimiters are for identifiers only. Delimiters cannot be used for keywords, whether or not they are marked as reserved in SQL Server.

• Quoted identifiers are delimited by double quotation marks ("):

  SELECT * FROM "Blanks in Table Name"
  

• Bracketed identifiers are delimited by brackets ([ ]):

  SELECT * FROM [Blanks In Table Name]
  

Quoted identifiers are valid only when the QUOTED_IDENTIFIER option is set to ON. By default, the Microsoft OLE DB Provider for SQL Server and SQL Server ODBC driver set QUOTED_IDENTIFIER ON when they connect. DB-Library does not set QUOTED_IDENTIFIER ON by default. Regardless of the interface used, individual applications or users may change the setting at any time. SQL Server provides a number of ways to specify this option. For example, in SQL Server Enterprise Manager and SQL Query Analyzer, the option can be set in a dialog box. In Transact-SQL, the option can be set at various levels using SET QUOTED_IDENTIFIER, the quoted identifier option of sp_dboption, or the user options option of sp_configure.

When QUOTED_IDENTIFIER is ON, SQL Server follows the SQL-92 rules for the use of double quotation marks and the single quotation mark (') in SQL statements:

• Double quotation marks can be used only to delimit identifiers. They cannot be used to delimit character strings.

  To maintain compatibility with existing applications, SQL Server does not fully enforce this rule. Character strings can be enclosed in double quotation marks if the string does not exceed the length of an identifier; this practice is not recommended.

• Single quotation marks must be used to enclose character strings. They cannot be used to delimit identifiers.

  If the character string contains an embedded single quotation mark, insert an additional single quotation mark in front of the embedded mark:

  SELECT * FROM "My Table"
  WHERE "Last Name" = 'O''Brien'
  

When QUOTED_IDENTIFIER is OFF, SQL Server follows these rules for the use of single and double quotation marks:

• Quotation marks cannot be used to delimit identifiers. Instead, use brackets as delimiters.
  
• Single or double quotation marks can be used to enclose character strings.

  If double quotation marks are used, embedded single quotation marks do not have to be denoted by two single quotation marks:

  SELECT * FROM [My Table]
  WHERE [Last Name] = "O'Brien"
  

Delimiters in brackets can always be used, regardless of the setting of QUOTED_IDENTIFIER.

Rules for Delimited Identifiers

The rules for the format of delimited identifiers are:

1. Delimited identifiers can contain the same number of characters as regular identifiers (from 1 through 128 characters, not including the delimiter characters). Local temporary table identifiers can be a maximum of 116 characters.
   
2. The body of the identifier can contain any combination of characters in the current code page except the delimiting characters themselves. For example, delimited identifiers can contain spaces, any characters valid for regular identifiers, and any of the following characters:

   tilde (~)	hyphen (-)
   exclamation point (!)	left brace ({)
   percent (%)	right brace (})
   caret (^)	apostrophe (')
   ampersand (&)	period (.)
   left parenthesis (()	backslash (\)
   right parenthesis ())	accent grave (`)

These examples use quoted identifiers for table names and column names. Both methods for specifying delimited identifiers are shown:

SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE "$Employee Data"
(
 "^First Name"   varchar(25) NOT NULL,
 "^Last Name"   varchar(25) NOT NULL,
 "^Dept ID"   int
)
-- INSERT statements go here.
SET QUOTED_IDENTIFIER OFF
GO
CREATE TABLE [^$Employee Data]
(
 [^First Name]   varchar(25) NOT NULL,
 [^Last Name]   varchar(25) NOT NULL,
 [^Dept ID]   int
)
-- INSERT statements go here.

After the $Employee Data and ^$Employee Data tables are created and data is entered, rows can be retrieved:

SET QUOTED_IDENTIFIER ON
GO
SELECT * 
FROM "$Employee Data"
SET QUOTED_IDENTIFIER OFF
GO
-- Or
SELECT *
FROM [^$Employee Data]

In this example, a table named table contains columns tablename, user, select, insert, and so on. Because TABLE, SELECT, INSERT, UPDATE, and DELETE are reserved keywords, the identifiers must be delimited each time the objects are accessed.

SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE "table"
(
 tablename char(128) NOT NULL,
 "USER"    char(128) NOT NULL,
 "SELECT"  char(128) NOT NULL,
 "INSERT"  char(128) NOT NULL,
 "UPDATE"  char(128) NOT NULL,
 "DELETE"  char(128) NOT NULL
)

If the SET QUOTED_IDENTIFIER option is not ON, the table and columns cannot be accessed unless bracket delimiters are used.

SET QUOTED_IDENTIFIER OFF
GO
SELECT * 
FROM "table"

Here is the result set:

Msg 170, Level 15, State 1
Line 1: Incorrect syntax near 'table'.

Here is the result set (using bracket delimiters):

SET QUOTED_IDENTIFIER OFF
GO
SELECT * 
FROM [table]

Delimiting Identifiers with Multiple Parts

When using qualified object names you may have to delimit more than one of the identifiers that make up the object name. Each identifier must be delimited individually, for example:

/* SQL-92 quoted identifier syntax */
SELECT *
FROM "My DB"."My#UserID"."My.Table"

-Or-

/* Transact-SQL bracketed identifier syntax */
/* Not available in SQL Server 6.5 or earlier */
SELECT *
FROM [My DB].[My#UserID].[My.Table]

There are some special rules regarding how you delimit multi-part stored procedure names in the ODBC CALL statement. For more information.

Using Identifiers as Parameters in SQL Server

Many system stored procedures, functions, and DBCC statements take object names as parameters. Some of these parameters accept multipart object names, while others accept only single-part names. Whether a single-part or multipart name is expected determines how a parameter is parsed and used internally by SQL Server.

Single-part Parameter Names

If the parameter is a single-part identifier, the name can be specified:

• Without quotation marks or delimiters.
  
• Enclosed in single quotation marks.
  
• Enclosed in double quotation marks.
  
• Enclosed in brackets.

For single-part names, the string inside the single quotation marks represents the object name. If delimiters are used inside single quotation marks, the delimiter characters are treated as part of the name.

If the name contains a period or another character that is not part of the character set defined for regular identifiers, you must enclose the object name in single quotation marks, double quotation marks, or brackets.

Multipart Parameter Names

Multipart names are qualified names that include the database or owner name in addition to the object name. SQL Server requires that when a multipart name is used as a parameter, the entire string that constitutes the multipart name must be enclosed in a set of single quotation marks.

EXEC MyProcedure @name = 'dbo.Employees'

If individual name parts require delimiters, each part of the name should be delimited separately as required. For example, if a name part contains a period, double quotation mark, or left or right bracket, use brackets or double quotation marks to delimit the part. Enclose the complete name in single quotation marks.

For example, the table name, tab.one, contains a period. To prevent the name from being interpreted as a three-part name, dbo.tab.one, delimit the table name part.

EXEC sp_help 'dbo.[tab.one]'

This example shows the same table name delimited with double quotation marks.

SET QUOTED_IDENTIFIER ON 
GO 
EXEC sp_help 'dbo."tab.one"'
GO 

This table lists some of the Transact-SQL functions, DBCC statements, and system stored procedures that use multipart names.