What is KEY in SQL?
A key in SQL is an attribute or a set of attribute which helps you uniquely identify a record or a row of data in a relation/ table.
A Primary key is a table attribute or column that identify every record present in a table uniquely.
There can be only one primary key attribute in a table and primary key column elements can’t be same or Null values.
Roll No. is the Primary Key in the above table.
It is the set of all columns which help to identify rows in a table uniquely.
A candidate key is a column or a set of columns that can qualify as a primary key in the database. There can be multiple candidate keys in a table and only one candidate key can qualify as the primary key.
It is a set of two or more attributes that helps to identify each tuple in a table uniquely. Any key such as super, primary, or candidate key etc. can be called composite key if it has more than one attribute.
A foreign key is a key used to link two tables together. This is sometimes also called a referencing key.