In SQL, the ADD COLUMN statement is used to add a new column to an existing table. This statement modifies the structure of the table and can be used to add a new column that contains a specific data type and constraints.
The basic syntax for the ADD COLUMN statement is as follows:
ALTER TABLE table_name ADD COLUMN column_name data_type constraints;
Here, the ALTER TABLE keyword is used to specify the name of the table that you want to modify. ADD COLUMN is used to add a new column to the table. column_name is the name of the new column that you want to add, and data_type is the data type that you want to assign to the new column. You can also add constraints to the new column, such as NOT NULL or UNIQUE.
Let’s take an example of adding a new column to an existing table:
ALTER TABLE employees ADD COLUMN email VARCHAR(255) UNIQUE;
In this example, we’re adding a new column called email to the employees table, with a data type of VARCHAR(255) and a constraint of UNIQUE. This means that every email value in the new column must be unique and cannot be null.
It’s important to note that when you add a new column to an existing table, the new column will be added to the end of the table by default. You can use the AFTER keyword to specify the position of the new column within the table.
ALTER TABLE employees ADD COLUMN phone_number VARCHAR(20) AFTER email;
In this example, we’re adding a new column called phone_number to the employees table, with a data type of VARCHAR(20) and it is placed after the email column.
In summary, the ADD COLUMN statement is a powerful SQL command that allows you to modify the structure of an existing table by adding a new column with a specific data type and constraints. This statement can help you to organize and manipulate your data more effectively.