ALTER TABLE Statement

The ALTER TABLE statement is used to modify an existing table definition in a SQL Server database. It allows you to add, delete, or modify columns, constraints, and other table properties.

Syntax

The basic syntax for ALTER TABLE is as follows:

ALTER TABLE table_name
ADD column_name datatype [constraints],
    ...
[CONSTRAINT constraint_name] { PRIMARY KEY | UNIQUE | FOREIGN KEY | CHECK } (column_name, ...),
    ...
[WITH ( options )];

Or to modify/drop existing elements:

ALTER TABLE table_name
{ ADD | ALTER | DROP } { COLUMN | CONSTRAINT | INDEX } ...;

Common Operations

Adding a Column

To add a new column to an existing table:

ALTER TABLE Customers
ADD Email VARCHAR(255);

Modifying a Column

To change the data type or size of an existing column:

ALTER TABLE Products
ALTER COLUMN Price DECIMAL(10, 2);

Note: Changing a column's data type can result in data loss or errors if the existing data cannot be converted to the new data type. Ensure you have backups and test thoroughly.

Dropping a Column

To remove a column from a table:

ALTER TABLE Orders
DROP COLUMN ShippingDate;

Important: Dropping a column is irreversible and will delete all data stored in that column.

Adding a Constraint

To add a new constraint, such as a UNIQUE constraint:

ALTER TABLE Employees
ADD CONSTRAINT UQ_EmployeeID UNIQUE (EmployeeID);

Dropping a Constraint

To remove an existing constraint:

ALTER TABLE Orders
DROP CONSTRAINT FK_CustomerID;

Key Considerations

  • Data Integrity: Be cautious when altering columns that contain data. Ensure that any changes (like data type modifications or dropping columns) do not compromise data integrity.
  • Dependencies: Before dropping a column or constraint, check for any dependencies (e.g., foreign key relationships, indexes, views, stored procedures) that might be affected.
  • Performance: Large tables can take a significant amount of time to alter, potentially locking the table. Plan your alterations during periods of low database activity.
  • Permissions: You need appropriate permissions (e.g., `ALTER` permission on the table or schema) to execute ALTER TABLE statements.

Related Topics