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Database identifiers

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Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW) SQL analytics endpoint in Microsoft Fabric Warehouse in Microsoft Fabric

The database object name is referred to as its identifier. Everything in SQL Server can have an identifier. Servers, databases, and database objects, such as tables, views, columns, indexes, triggers, procedures, constraints, and rules, can have identifiers. Identifiers are required for most objects, but are optional for some objects such as constraints.

An object identifier is created when the object is defined. The identifier is then used to reference the object. For example, the following statement creates a table with the identifier TableX, and two columns with the identifiers KeyCol and Description:

CREATE TABLE TableX (
    KeyCol INT PRIMARY KEY,
    Description NVARCHAR(80)
);

This table also has an unnamed constraint. The PRIMARY KEY constraint has no identifier.

The collation of an identifier depends on the level at which it's defined. Identifiers of instance-level objects, such as logins and database names, are assigned the default collation of the instance. Identifiers of objects in a database, such as tables, views, and column names, are assigned the default collation of the database. For example, two tables with names that differ only in case can be created in a database that has case-sensitive collation, but can't be created in a database that has case-insensitive collation.

Note

The names of variables, or the parameters of functions and stored procedures must comply with the rules for Transact-SQL identifiers.

Classes of identifiers

There are two classes of identifiers:

  • Regular identifiers comply with the rules for the format of identifiers. Regular identifiers aren't delimited when they're used in Transact-SQL statements.

    USE AdventureWorks2022;
GO

SELECT *
FROM HumanResources.Employee
WHERE NationalIDNumber = 153479919;

  • Delimited identifiers are enclosed in double quotation marks (") or brackets ([ and ]). Identifiers that comply with the rules for the format of identifiers might not be delimited. For example:

    USE AdventureWorks2022;
GO

SELECT *
FROM [HumanResources].[Employee] --Delimiter is optional.
WHERE [NationalIDNumber] = 153479919 --Delimiter is optional.

Identifiers that don't comply with all the rules for identifiers must be delimited in a Transact-SQL statement. For example:

USE AdventureWorks2022;
GO

--Identifier contains a space and uses a reserved keyword.
CREATE TABLE [SalesOrderDetail Table] (
    [Order] INT NOT NULL,
    [SalesOrderDetailID] INT IDENTITY(1, 1) NOT NULL,
    [OrderQty] SMALLINT NOT NULL,
    [ProductID] INT NOT NULL,
    [UnitPrice] MONEY NOT NULL,
    [UnitPriceDiscount] MONEY NOT NULL,
    [ModifiedDate] DATETIME NOT NULL,
    CONSTRAINT [PK_SalesOrderDetail_Order_SalesOrderDetailID] PRIMARY KEY CLUSTERED (
        [Order] ASC,
        [SalesOrderDetailID] ASC
    )
);
GO

SELECT *
FROM [SalesOrderDetail Table] --Identifier contains a space and uses a reserved keyword.
WHERE [Order] = 10; --Identifier is a reserved keyword.

Both regular and delimited identifiers must contain from 1 through 128 characters. For local temporary tables, the identifier can have a maximum of 116 characters.

Rules for regular identifiers

The names of variables, functions, and stored procedures must comply with the following rules for Transact-SQL identifiers.

  1. The first character must be one of the following items:

    • A letter as defined by the Unicode Standard 3.2. The Unicode definition of letters includes Latin characters from a through z, from A through Z, and also letter characters from other languages.

    • The underscore (_), at sign (@), or number sign (#).

      Certain symbols at the beginning of an identifier have special meaning in SQL Server. A regular identifier that starts with the at sign always denotes a local variable or parameter and can't be used as the name of any other type of object. An identifier that starts with a number sign denotes a temporary table or procedure. An identifier that starts with double number signs (##) denotes a global temporary object. Although the number sign or double number sign characters can be used to begin the names of other types of objects, we don't recommend this practice.

      Some Transact-SQL functions have names that start with double at signs (@@). To avoid confusion with these functions, you shouldn't use names that start with @@.

  2. Subsequent characters can include the following list:

    • Letters as defined in the Unicode Standard 3.2.

    • Decimal numbers from either Basic Latin or other national scripts.

    • The at sign (@), dollar sign ($), number sign (#), or underscore (_).

  3. The identifier must not be a Transact-SQL reserved word. SQL Server reserves both the uppercase and lowercase versions of reserved words. When identifiers are used in Transact-SQL statements, the identifiers that don't comply with these rules must be delimited by double quotation marks or brackets. The words that are reserved depend on the database compatibility level. This level can be set by using the ALTER DATABASE compatibility level statement.

  4. Embedded spaces or special characters aren't allowed.

  5. Supplementary characters aren't allowed.

When identifiers are used in Transact-SQL statements, the identifiers that don't comply with these rules must be delimited by double quotation marks or brackets.

Note

Some rules for the format of regular identifiers depend on the database compatibility level. This level can be set by using ALTER DATABASE compatibility level.

Catalog collation in Azure SQL Database

You can't change or set the logical server collation on Azure SQL Database. However, you can configure each database's collations separately for data in the database and for catalog. The catalog collation determines the collation for system metadata, such as object identifiers. Both collations can be specified independently when you create the database in the Azure portal, in T-SQL with CREATE DATABASE, in PowerShell with New-AzSqlDatabase.

For details and examples, see CREATE DATABASE. Specify a collation for the database (COLLATE) and a catalog collation for system metadata and object identifiers (CATALOG_COLLATION).

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