The correct answer is: C. Both of above.
A primary key is a column or set of columns in a table that uniquely identifies each row in the table. A primary key must contain unique values and cannot contain null values.
For example, in a table of employees, the employee ID column could be the primary key. This is because each employee has a unique employee ID, and no two employees can have the same employee ID.
Another example is a table of products. The product ID column could be the primary key. This is because each product has a unique product ID, and no two products can have the same product ID.
It is important to have a primary key in a table because it allows you to uniquely identify each row in the table. This is necessary for many database operations, such as joins and queries.
Here are some additional details about each option:
- Option A: Not Null. A primary key cannot contain null values. This means that every row in the table must have a value in the primary key column.
- Option B: No Duplicates. A primary key cannot contain duplicate values. This means that no two rows in the table can have the same value in the primary key column.
I hope this helps! Let me know if you have any other questions.