Use `ALTER TABLE table_name ADD column_name data_type;`
Example: `ALTER TABLE employees ADD age INT;`
Add multiple columns: `ALTER TABLE table_name ADD column1 data_type, ADD column2 data_type;`
Example: `ALTER TABLE employees ADD department VARCHAR(50), ADD salary DECIMAL(10,2);`
For SQL Server, use: `ALTER TABLE table_name ADD column_name data_type;`
For MySQL, use: `ALTER TABLE table_name ADD column_name data_type;`
For PostgreSQL, use: `ALTER TABLE table_name ADD COLUMN column_name data_type;`
For Oracle, use: `ALTER TABLE table_name ADD column_name data_type;`
To add a column with a default value: `ALTER TABLE table_name ADD column_name data_type DEFAULT value;`
To add a column that allows nulls: `ALTER TABLE table_name ADD column_name data_type NULL;`
To add a column that does not allow nulls: `ALTER TABLE table_name ADD column_name data_type NOT NULL;`