CREATE DEFAULT (Transact-SQL)

Artikkeli
12/29/2022

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance

Creates an object called a default. When bound to a column or an alias data type, a default specifies a value to be inserted into the column to which the object is bound (or into all columns, if of an alias data type), when no value is explicitly supplied during an insert.

Important

This feature will be removed in a future version of SQL Server. Avoid using this feature in new development work, and plan to modify applications that currently use this feature. Instead, use default definitions created using the DEFAULT keyword of ALTER TABLE or CREATE TABLE.

Transact-SQL syntax conventions

Syntax

  
CREATE DEFAULT [ schema_name . ] default_name   
AS constant_expression [ ; ]

Arguments

schema_name
The name of the schema to which the default belongs.

default_name
The name of the default. Default names must conform to the rules for identifiers. Specifying the default owner name is optional.

constant_expression
An expression that contains only constant values (it can't include the names of any columns or other database objects). You can use any constant, built-in function, or mathematical expression, except ones that contain alias data types. User-defined functions can't be used. Enclose character and date constants in single quotation marks ('); monetary, integer, and floating-point constants don't require quotation marks. Binary data must be preceded by 0x, and monetary data must be preceded by a dollar sign ($). The default value must be compatible with the column data type.

Remarks

You can only create a default name in the current database. Within a database, default names must be unique by schema. When you create a default, use sp_bindefault to bind it to a column or to an alias data type.

If the default isn't compatible with the column to which it's bound, SQL Server generates an error message when trying to insert the default value. For example, N/A can't be used as a default for a numeric column.

If the default value is too long for the column to which it's bound, the value is truncated.

CREATE DEFAULT statements can't be combined with other Transact-SQL statements in a single batch.

A default must be dropped before creating a new one of the same name. And, the default must be unbound by executing sp_unbindefault before it's dropped.

If a column has both a default and a rule associated with it, the default value must not violate the rule. A default that conflicts with a rule is never inserted, and SQL Server generates an error message each time it attempts to insert the default.

When bound to a column, a default value is inserted when:

A value isn't explicitly inserted.
Either the DEFAULT VALUES or DEFAULT keywords are used with INSERT to insert default values.

If you specify NOT NULL when creating a column and you don't create a default for it, an error message generates when a user fails to make an entry in that column. The following table illustrates the relationship between the existence of a default and the definition of a column as NULL or NOT NULL. The entries in the table show the result.

Column definition	No entry, no default	No entry, default	Enter NULL, no default	Enter NULL, default
NULL	NULL	default	NULL	NULL
NOT NULL	Error	default	error	error

To rename a default, use sp_rename. For a report on a default, use sp_help.

Permissions

To use CREATE DEFAULT, at a minimum, a user must have CREATE DEFAULT permission in the current database and ALTER permission on the schema in which the default is being created.

Examples

A. Creating a simple character default

The following example creates a character default called unknown.

USE AdventureWorks2022;  
GO  
CREATE DEFAULT phonedflt AS 'unknown';

B. Binding a default

The following example binds the default created in example A. The default takes effect only if no entry is specified for the Phone column of the Contact table.

Note

Omitting any entry is different from explicitly stating NULL in an INSERT statement.

Because a default named phonedflt doesn't exist, the following Transact-SQL statement fails. This example is for illustration only.

USE AdventureWorks2022;  
GO  
sp_bindefault 'phonedflt', 'Person.PersonPhone.PhoneNumber';

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