Primary Key Constraints

A primary key constraint on a column instructs the database engine to keep the entries in a column unique. For example, if we were to create a table with information about all the human beings on Earth, we could use the tongue print of human beings as unique identification. If tongue prints were in a column, it would be the primary key.

It is possible to have a duplicate tongue print; however, it is rare. In such a case, you could create a primary key across multiple columns. Therefore, you could combine the tongue print, fingerprint, and the retinal signature to make a primary key. In such a case, the combination of these values in these columns should be unique across the table. In other words, there may be a duplicate tongue print, a duplicate fingerprint, and a duplicate retinal signature in the table—the database engine will allow that. However, there cannot be a duplicate combination of all three. Alternatively, there can be no two human beings...