ALTER TABLE column_constraint (Transact-SQL)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Specifies the properties of a PRIMARY KEY, FOREIGN KEY, UNIQUE, or CHECK constraint that is part of a new column definition added to a table by using ALTER TABLE.
Transact-SQL syntax conventions
Syntax
Syntax for SQL Server, Azure SQL Database, Azure SQL Managed Instance, SQL database in Microsoft Fabric
[ CONSTRAINT constraint_name ]
{
[ NULL | NOT NULL ]
{ PRIMARY KEY | UNIQUE }
[ CLUSTERED | NONCLUSTERED ]
[ WITH FILLFACTOR = fillfactor ]
[ WITH ( index_option [, ...n ] ) ]
[ ON { partition_scheme_name (partition_column_name)
| filegroup | "default" } ]
| [ FOREIGN KEY ]
REFERENCES [ schema_name . ] referenced_table_name
[ ( ref_column ) ]
[ ON DELETE { NO ACTION | CASCADE | SET NULL | SET DEFAULT } ]
[ ON UPDATE { NO ACTION | CASCADE | SET NULL | SET DEFAULT } ]
[ NOT FOR REPLICATION ]
| CHECK [ NOT FOR REPLICATION ] ( logical_expression )
}
Syntax for Microsoft Fabric Warehouse
[ CONSTRAINT constraint_name ]
{
{ PRIMARY KEY | UNIQUE }
NONCLUSTERED
(column [ ASC | DESC ] [ ,...n ] )
NOT ENFORCED
| FOREIGN KEY
( column [ ,...n ] )
REFERENCES referenced_table_name [ ( ref_column [ ,...n ] ) ]
NOT ENFORCED
}
Arguments
CONSTRAINT
Specifies the start of the definition for a PRIMARY KEY, UNIQUE, FOREIGN KEY, or CHECK constraint.
constraint_name
Is the name of the constraint. Constraint names must follow the rules for identifiers, except that the name cannot start with a number sign (#). If constraint_name is not supplied, a system-generated name is assigned to the constraint.
NULL | NOT NULL
Specifies whether the column can accept null values. Columns that do not allow null values can be added only if they have a default specified. If the new column allows null values and no default is specified, the new column contains NULL for each row in the table. If the new column allows null values and a default definition is added with the new column, the WITH VALUES option can be used to store the default value in the new column for each existing row in the table.
If the new column does not allow null values, a DEFAULT definition must be added with the new column. The new column automatically loads with the default value in the new columns in each existing row.
When the addition of a column requires physical changes to the data rows of a table, such as adding DEFAULT values to each row, locks are held on the table while ALTER TABLE runs. This affects the ability to change the content of the table while the lock is in place. In contrast, adding a column that allows null values and does not specify a default value is a metadata operation only, and involves no locks.
When you use CREATE TABLE or ALTER TABLE, database and session settings influence and possibly override the nullability of the data type that is used in a column definition. We recommend that you always explicitly define noncomputed columns as NULL or NOT NULL or, if you use a user-defined data type, that you allow the column to use the default nullability of the data type. For more information, see CREATE TABLE.
PRIMARY KEY
Is a constraint that enforces entity integrity for a specified column or columns by using a unique index. Only one PRIMARY KEY constraint can be created for each table.
UNIQUE
Is a constraint that provides entity integrity for a specified column or columns by using a unique index.
CLUSTERED | NONCLUSTERED
Specifies that a clustered or nonclustered index is created for the PRIMARY KEY or UNIQUE constraint. PRIMARY KEY constraints default to CLUSTERED. UNIQUE constraints default to NONCLUSTERED.
If a clustered constraint or index already exists on a table, CLUSTERED cannot be specified. If a clustered constraint or index already exists on a table, PRIMARY KEY constraints default to NONCLUSTERED.
Columns that are of the ntext, text, varchar(max), nvarchar(max), varbinary(max), xml, or image data types cannot be specified as columns for an index.
In Microsoft Fabric Warehouse, NONCLUSTERED has no effect.
WITH FILLFACTOR =fillfactor
Specifies how full the Database Engine should make each index page used to store the index data. User-specified fill factor values can be from 1 through 100. If a value is not specified, the default is 0.
Important
Documenting WITH FILLFACTOR as the only index option that applies to PRIMARY KEY or UNIQUE constraints is maintained for backward compatibility, but will not be documented in this manner in future releases. Other index options can be specified in the index_option clause of ALTER TABLE.
ON { partition_scheme_name(partition_column_name) | filegroup | default }
Applies to: SQL Server 2008 (10.0.x) and later.
Specifies the storage location of the index created for the constraint. If partition_scheme_name is specified, the index is partitioned and the partitions are mapped to the filegroups that are specified by partition_scheme_name. If filegroup is specified, the index is created in the named filegroup. If "default" is specified or if ON is not specified at all, the index is created in the same filegroup as the table. If ON is specified when a clustered index is added for a PRIMARY KEY or UNIQUE constraint, the whole table is moved to the specified filegroup when the clustered index is created.
In this context, default, is not a keyword. It is an identifier for the default filegroup and must be delimited, as in ON default or ON [default]. If default is specified, the QUOTED_IDENTIFIER option must be ON for the current session. This is the default setting. For more information, see SET QUOTED_IDENTIFIER (Transact-SQL).
NOT ENFORCED
In Microsoft Fabric Warehouse, primary key, unique key, and foreign key constraints require NOT ENFORCED. Integrity of the constraints must be maintained by the application.
FOREIGN KEY REFERENCES
Is a constraint that provides referential integrity for the data in the column. FOREIGN KEY constraints require that each value in the column exists in the specified column in the referenced table.
schema_name
Is the name of the schema to which the table referenced by the FOREIGN KEY constraint belongs.
referenced_table_name
Is the table referenced by the FOREIGN KEY constraint.
ref_column
Is a column in parentheses referenced by the new FOREIGN KEY constraint.
ON DELETE { NO ACTION | CASCADE | SET NULL | SET DEFAULT }
Specifies what action happens to rows in the table that is altered, if those rows have a referential relationship and the referenced row is deleted from the parent table. The default is NO ACTION.
NO ACTION
The SQL Server Database Engine raises an error and the delete action on the row in the parent table is rolled back.
CASCADE
Corresponding rows are deleted from the referencing table if that row is deleted from the parent table.
SET NULL
All the values that make up the foreign key are set to NULL when the corresponding row in the parent table is deleted. For this constraint to execute, the foreign key columns must be nullable.
SET DEFAULT
All the values that comprise the foreign key are set to their default values when the corresponding row in the parent table is deleted. For this constraint to execute, all foreign key columns must have default definitions. If a column is nullable and there is no explicit default value set, NULL becomes the implicit default value of the column.
Do not specify CASCADE if the table will be included in a merge publication that uses logical records. For more information about logical records, see Group Changes to Related Rows with Logical Records.
The ON DELETE CASCADE cannot be defined if an INSTEAD OF trigger ON DELETE already exists on the table that is being altered.
For example, in the AdventureWorks2022 database, the ProductVendor table has a referential relationship with the Vendor table. The ProductVendor.VendorID foreign key references the Vendor.VendorID primary key.
If a DELETE statement is executed on a row in the Vendor table, and an ON DELETE CASCADE action is specified for ProductVendor.VendorID, the Database Engine checks for one or more dependent rows in the ProductVendor table. If any exist, the dependent rows in the ProductVendor table will be deleted, in addition to the row referenced in the Vendor table.
Conversely, if NO ACTION is specified, the Database Engine raises an error and rolls back the delete action on the Vendor row when there is at least one row in the ProductVendor table that references it.
ON UPDATE { NO ACTION | CASCADE | SET NULL | SET DEFAULT }
Specifies what action happens to rows in the table altered when those rows have a referential relationship and the referenced row is updated in the parent table. The default is NO ACTION.
NO ACTION
The Database Engine raises an error, and the update action on the row in the parent table is rolled back.
CASCADE
Corresponding rows are updated in the referencing table when that row is updated in the parent table.
SET NULL
All the values that make up the foreign key are set to NULL when the corresponding row in the parent table is updated. For this constraint to execute, the foreign key columns must be nullable.
SET DEFAULT
All the values that make up the foreign key are set to their default values when the corresponding row in the parent table is updated. For this constraint to execute, all foreign key columns must have default definitions. If a column is nullable and there is no explicit default value set, NULL becomes the implicit default value of the column.
Do not specify CASCADE if the table will be included in a merge publication that uses logical records. For more information about logical records, see Group Changes to Related Rows with Logical Records.
ON UPDATE CASCADE, SET NULL, or SET DEFAULT cannot be defined if an INSTEAD OF trigger ON UPDATE already exists on the table that is being altered.
For example, in the AdventureWorks2022 database, the ProductVendor table has a referential relationship with the Vendor table. The ProductVendor.VendorID foreign key references the Vendor.VendorID primary key.
If an UPDATE statement is executed on a row in the Vendor table and an ON UPDATE CASCADE action is specified for ProductVendor.VendorID, the Database Engine checks for one or more dependent rows in the ProductVendor table. If any exist, the dependent row in the ProductVendor table will be updated, in addition to the row referenced in the Vendor table.
Conversely, if NO ACTION is specified, the Database Engine raises an error and rolls back the update action on the Vendor row when there is at least one row in the ProductVendor table that references it.
NOT FOR REPLICATION
Applies to: SQL Server 2008 (10.0.x) and later.
Can be specified for FOREIGN KEY constraints and CHECK constraints. If this clause is specified for a constraint, the constraint is not enforced when replication agents perform insert, update, or delete operations.
CHECK
Is a constraint that enforces domain integrity by limiting the possible values that can be entered into a column or columns.
logical_expression
Is a logical expression used in a CHECK constraint and returns TRUE or FALSE. logical_expression used with CHECK constraints cannot reference another table but can reference other columns in the same table for the same row. The expression cannot reference an alias data type.
Remarks
When FOREIGN KEY or CHECK constraints are added, all existing data is verified for constraint violations unless the WITH NOCHECK option is specified. If any violations occur, ALTER TABLE fails and an error is returned. When a new PRIMARY KEY or UNIQUE constraint is added to an existing column, the data in the column or columns must be unique. If duplicate values are found, ALTER TABLE fails. The WITH NOCHECK option has no effect when PRIMARY KEY or UNIQUE constraints are added.
Each PRIMARY KEY and UNIQUE constraint generates an index. The number of UNIQUE and PRIMARY KEY constraints cannot cause the number of indexes on the table to exceed 999 nonclustered indexes and 1 clustered index. Foreign key constraints do not automatically generate an index. However, foreign key columns are frequently used in join criteria in queries by matching the column or columns in the foreign key constraint of one table with the primary or unique key column or columns in the other table. An index on the foreign key columns enables the Database Engine to quickly find related data in the foreign key table.
Examples
For examples, see ALTER TABLE (Transact-SQL).