*This post may contain affiliate links. As an Amazon Associate we earn from qualifying purchases.
During your IT interview, you may be asked about the definition of a key in SQL. This is actually one of the most commonly asked questions, so take a look at the main things you should mention to make sure you provide a complete answer and make a good impression.
What Official Definition of a Key in SQL?
According to the official definition of a key in SQL, a key is a subclass of columns integrated into a table which is meant to allow a certain row to be identified uniquely. This means that a key may be more than one single column.
The main condition is that every row has a unique value for the key. However, this may mean that the row has a unique value included in one column or a unique combination of values included in several columns.
Is a Key Allowed to Have Null Values in SQL?
According to the official definition of a key in SQL, it is not permitted to have a null or a null-able value. If a key has more columns than those deemed necessary to define each of the rows included in the table, it is then called a Super-Key.
On the other hand, if the key has the exact amount of columns that are necessary to identify each row in the table, it is then called a Minimal Super-Key. In fact, it is very important that each table features at least one Minimal Super-Key.
Types of Keys in SQL
According to the official definition of a key in SQL, there are quite a few types of keys that you should know about. Take a look!
As we have mentioned above, Super Keys are unique combinations of fields included in a table that can uniquely identify one single record from that table.
According to the complete definition of a key in SQL, Candidate Keys are a?sub-type?of Super Keys. They consist of one single field or of the minimum combination of fields that can uniquely identify a certain record featured in the table. Each table is required to have a minimum of one Candidate Key, but it can have as many as necessary.
A Primary Key is the most important type of Candidate Key. It is usually the main reference key for a certain table. It is called a Primary Key because it can be used as the main reference for that table and because it can be used in the database to establish various relationships with other tables. Please note that all Primary Keys must be unique and they may never be NULL.
Foreign Keys are used to establish a relationship between two separate tables. A Foreign Key is a special type of Primary Key that belongs to one table, but that also appears as a field in a second table. It can be used to define the relationship between the two tables. Since it is a subtype of Primary Keys, Foreign Keys may never be null and must always be unique.
A table may have more than one key that could be used as a Primary Key. While only one will be chosen as the table?s Primary Key, all the others will be known as Secondary Keys. Think of these as potential placeholders or alternatives for Primary Keys.
Simple Keys are comprised of a single field in a table that can uniquely identify a certain record. A Simple Key may never be divided into two or more other fields, it must be solitary and unique.
As opposed to Simple Keys, Compound Keys are comprised of more than one field in the table but they identify a certain record uniquely. Also, each field included in a Compound Key is a Simple Key.
Just like Compound Keys, Composite Keys are made up of more than one field in the table and they identify one single record uniquely. The main difference between Compound Keys and Composite Keys is that the attributes that are included in the Composite Keys are not Simple Keys.
We hope that the information we have provided was useful to you and that you can now answer any question related to the definition of a key in SQL. Good luck on your job meeting from the IT Interview Guide team!