Relational Database Keys

Whether you are a musician or not, you will find that there are some common keys that you will use in almost any musical style. These keys include the Primary Key, the Foreign Key, the Candidate Key and the Composite Key.

Primary key

Using a Primary Key can be a useful method of ensuring data integrity and stability of your database. It will also help you identify each record in your table.

A primary key is a set of columns that uniquely identifies each row in your table. The key column should not contain null values. If the column contains a null value, you cannot update the table with that value.

A Primary Key can be a composite key or an explicit primary key. Composite keys contain two or more columns. Explicit primary keys are used to create tables that contain both types of keys.

A candidate key is a set of keys that could possibly be used as the primary key. For example, if you have a table that contains license plate numbers, you could create a candidate key of "plateNo" and "state". This key could identify the same license plate number in different states.

Foreign key

Using foreign key in relational database allows you to relate tables in a meaningful manner. Without foreign key, you cannot maintain the integrity of data between two different instances of an entity.

Foreign key is a table or a combination of columns that references the primary key of another table. These columns must have values that match the values in the primary key of the second table. If a column in the second table contains a value that is not in the primary key, the column will become null.

A foreign key referential action is a standard operation for foreign keys. It is similar to the ON DELETE SET NULL action, but a foreign key referential action only happens when a table is altered.

Composite key

Basically, a composite key is a mix of two or more columns that is used to uniquely identify each row of a table. These columns can be of different data types.

Depending on the type of composite key you choose, you may need to expand the column set if you change the data. In addition, the database server may return an error if the row does not match. You can also alter the columns in the composite key using alter commands.

Generally, a composite key will be referenced in other tables as a foreign key. This means that you will need to update your foreign keys if you change the format of your data.

Candidate key

Often referred to as the "minimal" of the super key, the candidate key is a set of attributes that uniquely identifies a row in a table. It may also qualify as the primary key.

It is used to ensure the integrity of data. It also helps to differentiate between similar entries in a database. A candidate key can be a set of columns or a combination of columns. The goal is to prevent the creation of duplicate rows.

A candidate key may contain attributes that are null or redundant. In general, it should not contain any null values. It should also contain the minimum number of attributes.