ALTER TABLE table_constraint (Transact-SQL)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Warehouse in Microsoft Fabric
SQL database in Microsoft Fabric
Specifies the properties of a PRIMARY KEY, UNIQUE, FOREIGN KEY, a CHECK constraint, or a DEFAULT definition added to a table by using ALTER TABLE.
Transact-SQL syntax conventions
Syntax
[ CONSTRAINT constraint_name ]
{
{ PRIMARY KEY | UNIQUE }
[ CLUSTERED | NONCLUSTERED ]
(column [ ASC | DESC ] [ ,...n ] )
[ WITH FILLFACTOR = fillfactor
[ WITH ( <index_option>[ , ...n ] ) ]
[ ON { partition_scheme_name ( partition_column_name ... )
| filegroup | "default" } ]
| FOREIGN KEY
( column [ ,...n ] )
REFERENCES referenced_table_name [ ( ref_column [ ,...n ] ) ]
[ ON DELETE { NO ACTION | CASCADE | SET NULL | SET DEFAULT } ]
[ ON UPDATE { NO ACTION | CASCADE | SET NULL | SET DEFAULT } ]
[ NOT FOR REPLICATION ]
| CONNECTION
( { node_table TO node_table }
[ , {node_table TO node_table }]
[ , ...n ]
)
[ ON DELETE { NO ACTION | CASCADE } ]
| DEFAULT constant_expression FOR column [ WITH VALUES ]
| CHECK [ NOT FOR REPLICATION ] ( logical_expression )
}
Arguments
CONSTRAINT
Specifies the start of a definition for a PRIMARY KEY, UNIQUE, FOREIGN KEY, or CHECK constraint, or a DEFAULT.
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.
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.
column
Is a column or list of columns specified in parentheses that are used in a new constraint.
[ ASC | DESC ]
Specifies the order in which the column or columns participating in table constraints are sorted. The default is ASC.
WITH FILLFACTOR =fillfactor
Specifies how full the Database Engine should make each index page used to store the index data. User-specified fillfactor values can be from 1 through 100. If a value is not specified, the default is 0.
Important
Documenting WITH FILLFACTOR = 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.
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 exist in the specified column in the referenced table.
referenced_table_name
Is the table referenced by the FOREIGN KEY constraint.
ref_column
Is a column or list of columns 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.
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, as well as 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.
CONNECTION Specifies the pair of node tables that the given edge constraint is allowed to connect. ON DELETE specifies what happens to the rows in the edge table, when the nodes which were connected via the edge(s) in this edge table are deleted.
DEFAULT
Specifies the default value for the column. DEFAULT definitions can be used to provide values for a new column in the existing rows of data. DEFAULT definitions cannot be added to columns that have a timestamp data type, an IDENTITY property, an existing DEFAULT definition, or a bound default. If the column has an existing default, the default must be dropped before the new default can be added. If a default value is specified for a user-defined type column, the type should support an implicit conversion from constant_expression to the user-defined type. To maintain compatibility with earlier versions of SQL Server, a constraint name can be assigned to a DEFAULT.
constant_expression
Is a literal value, a NULL, or a system function that is used as the default column value. If constant_expression is used in conjunction with a column defined to be of a Microsoft .NET Framework user-defined type, the implementation of the type must support an implicit conversion from the constant_expression to the user-defined type.
FOR column
Specifies the column associated with a table-level DEFAULT definition.
WITH VALUES
When adding a column AND a DEFAULT constraint, if the column allows NULLS using WITH VALUES will, for existing rows, set the new column's value to the value given in DEFAULT constant_expression.
If the column being added does not allow NULLS, for existing rows, the column's value will always be set to the value given in the DEFAULT constant expression.
Starting in SQL Server 2012 this may be a meta data operation adding-not-null-columns-as-an-online-operation.
If this is used when the related column isn't also being added then it has no effect.
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.
SQL Server 2022 (16.x) introduces resumable operations for adding table constraints for primary key and unique key constraints. For more information on enabling and using resumable ALTER TABLE ADD CONSTRAINT operations, see Resumable add table constraints.
Examples
For examples, see ALTER TABLE (Transact-SQL).