super-key: Any key that has more columns than necessary to uniquely identify each row in the table is called a super-key (think of it as a super-set).
candidate key: if the key has the minimum amount of columns necessary to uniquely identify each row then it is called a minimal super-key. A minimal super-key is also known as a candidate key, and there must be one or more candidate keys in a table.
foreign key: While unique and primary keys both enforce uniqueness on the column(s) of one table, foreign keys define a relationship between two tables. A foreign key identifies a column or group of columns in one (referencing) table that refers to a column or group of columns in another (referenced) table
Can a table have multiple unique, foreign, and/or primary keys?
A table can have multiple unique and foreign keys. However, a table can have only one primary key.
Can a unique key have NULL values? Can a primary key have NULL values?
Unique key columns are allowed to hold NULL values. The values in a primary key column, however, can never be NULL.
Can a foreign key reference a non-primary key?
Yes, a foreign key can actually reference a key that is not the primary key of a table. But, a foreign key must reference a unique key.
Can a foreign key contain null values?
Yes, a foreign key can hold NULL values. Because foreign keys can reference unique, non-primary keys – which can hold NULL values – this means that foreign keys can themselves hold NULL values as well.
natural key: A natural key is a key composed of columns that actually have a logical relationship to other columns within a table.
simple key: In a database table, a simple key is just a single attribute (which is just a column) that can uniquely identify a row. So, any single column in a table that can uniquely identify a row is a simple key. The reason it’s called a simple key is because of the fact that it is simple in the sense that it’s just composed of one column (as opposed to multiple columns) and that’s it.
secondary key: A given table may have more than just one choice for a primary key. Basically, there may be another column (or combination of columns for a multi-column primary key) that qualify as primary keys. Any combination of column(s) that may qualify to be a primary key are known as candidate keys. This is because they are considered candidates for the primary key. And the options that are not selected to be the primary key are known as secondary keys.