Question

I created a table like this:

CREATE TABLE Persons
(
    P_Id int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Address varchar(255),
    City varchar(255),
    PRIMARY KEY (P_Id)
)

but I can’t understand the purpose of the primary key

PRIMARY KEY (P_Id)

in that table.

Answer 1

From MSDN:

A table typically has a column or combination of columns that contain
values that uniquely identify each row in the table. This column, or
columns, is called the primary key (PK) of the table and enforces the
entity integrity of the table. You can create a primary key by
defining a PRIMARY KEY constraint when you create or modify a table.