Question

what is a primary key? what is important in a table ?how many primary key can be set in table

Answer 1

Primary key is a column or combination of columns that uniquely defines a row in a table of a relational database.

Table is the data representation of rows and columns. It help to easily access of the data or information.

Only one primary key can be set in a Table.

Answer 2

In an SQL server database,
a Primary Key is a value that uniquely defines an entry. For example,
if there are three students, "Adam", "Bethany", "Cindy", we can create a primary key for them, in this case roll number. Like this:

Roll     Name
1         Adam
2         Bethany
3         Cindy

In this case, Roll is the primary key, as it identifies Name, uniquely. In SQL Server and SharePoint, the primary key is shown as a yellow key.