SQL ALTER TABLE: Adding Primary Key Constraint

The SQL statement ALTER TABLE customer ADD CONSTRAINT pk_id PRIMARY KEY (customer_id) will add a primary key constraint to the customer table, ensuring that the customer_id column is unique and not null.

Explanation:

• ALTER TABLE customer: Modifies the existing customer table.
• ADD CONSTRAINT pk_id: Adds a constraint named pk_id.
• PRIMARY KEY (customer_id): Specifies that customer_id is the primary key column.

Therefore, the correct answer is C. It will add a primary key constraint column customer id in customer table.