SQL ALTER TABLE: Adding a 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 using the 'customer_id' column. This means:

• A primary key constraint ensures that each row in the table has a unique value for the specified column (in this case, 'customer_id').

• The 'pk_id' is the name of the constraint.

• This statement will not add a new column to the table. It will simply add a constraint to an existing column.

Therefore, the correct answer is A.

Incorrect answers:

• B: This statement adds a primary key constraint, not removes it.
• C: This statement does not add a new column, it adds a constraint to an existing column.
• D: The statement has a specific purpose and will add a primary key constraint.