What is candidate key, and how many other keys are present in sql?

Hi Ayaz! Thanks for reaching out. There are several type of keys we can use in MySQL. The ones that are most important for database theory have been presented in this course. The rest are useful for database designers and are not a must for the data scientist toolkit. Nevertheless, a candidate key is a set of one or several fields/columns in a data table. Potentially, it can identify a unique record (i.e. row) in that table. Therefore, the candidate key (and that's why it is called candidate) could potentially identify a primary key. Just, a primary key cannot contain null values, while candidate keys can. Hope this helps. Best, Martin