In SQL Server, the
ALTER TABLE statement is used to modify an existing table's structure.
One of the common operations performed with the ALTER TABLE command is adding a new column to an existing table.
This allows you to extend the schema of your database and store additional data.
The syntax for adding a column to an existing table in SQL Server is as follows:
ALTER TABLE table_name
ADD column_name data_type [column_constraint];
Let's break down the components of this syntax:
ALTER TABLE: Specifies that you want to modify an existing table.
table_name: The name of the table to which you want to add the column.
ADD: Indicates that you want to add a new column.
column_name: The name of the new column you want to add.
data_type: The data type of the new column, such as
column_constraint (optional): Any constraints or attributes you want to apply to the new column, such as NOT NULL or DEFAULT values.
Here are a few examples to illustrate how to use the
ALTER TABLE ADD COLUMN command:
Example 1: Adding a basic column.
ALTER TABLE employees
ADD email VARCHAR(100);
This adds a new column named "email" of type VARCHAR with a maximum length of 100 characters to the "employees" table.
Example 2: Adding a column with a NOT NULL constraint.
ALTER TABLE customers
ADD phone_number VARCHAR(15) NOT NULL;
Example 3: Adding a column with a default value.
ALTER TABLE orders
ADD order_status VARCHAR(20) DEFAULT 'pending';
This adds a new column named "order_status" of type VARCHAR with a maximum length of 20 characters to the "orders" table. The column is assigned a default value of 'pending'. If a value is not explicitly provided for this column during an INSERT operation, it will default to 'pending'.
Remember, when adding a column to an existing table, you should consider the impact on the existing data and any related queries or applications.