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ALTER TABLE table_name
ADD CONSTRAINT constraint_name
{ PRIMARY KEY (column_list)
| UNIQUE (column_list)
| FOREIGN KEY (column_list) REFERENCES ref_table (ref_column_list)
| CHECK (expression)
| DEFAULT default_value FOR column_name };
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Parameter	Description
table_name	Name of the table to modify.
constraint_name	Identifier for the new constraint.
column_list	Comma‑separated list of columns the constraint applies to.
ref_table	Referenced table for a foreign key.
ref_column_list	Columns in the referenced table.
expression	Boolean expression for a CHECK constraint.
default_value	Literal value for a DEFAULT constraint.

Primary Key

ALTER TABLE Employees
ADD CONSTRAINT PK_Employees PRIMARY KEY (EmployeeID);
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Foreign Key

ALTER TABLE Orders
ADD CONSTRAINT FK_Orders_Customers
FOREIGN KEY (CustomerID) REFERENCES Customers (CustomerID);
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Check Constraint

ALTER TABLE Products
ADD CONSTRAINT CHK_Price CHECK (Price > 0);
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Unique Constraint

ALTER TABLE Users
ADD CONSTRAINT UQ_Username UNIQUE (Username);
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Default Constraint

ALTER TABLE Orders
ADD CONSTRAINT DF_OrderDate DEFAULT GETDATE() FOR OrderDate;
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• Constraints enforce data integrity and cannot be added if existing data violates the rule.
• When adding a foreign key, ensure referenced columns have an appropriate primary or unique key.
• Use descriptive names for constraints to improve maintainability.
• SQL Server supports disabling constraints temporarily with NOCHECK.