Primary key in DBMS

Definition: A primary is a column or set of columns in a table that uniquely identifies tuples (rows) in that table.

Example:
Student Table

In the above Student table, the Stu_Id column uniquely identifies each row of the table.

Note:

• We denote the primary key by underlining the column name.
• The value of primary key should be unique for each row of the table. Primary key column cannot contain duplicate values.
• Primary key column should not contain nulls.
• Primary keys are not necessarily to be a single column; more than one column can also be a primary key for a table. For e.g. {Stu_Id, Stu_Name} collectively can play a role of primary key in the above table, but that does not make sense because Stu_Id alone is enough to uniquely identifies rows in a table then why to  make things complex. Having that said, we should choose more than one columns as primary key only when there is no single column that can play the role of primary key.

How to choose a primary key?
There are two ways: Either to create a column and let database automatically have numbers in increasing order for each row or choose a column yourself making sure that it does not contain duplicates and nulls. For e.g. in the above Student table, The Stu_Name column cannot be a primary key as more than one people can have same name, similarly the Stu_Age column cannot play a primary key role as more than one persons can have same age.