PRIMARY KEY

Example

CREATE TABLE Employees (
    ID int,
    Name varchar(255),
    Age int,
    Address varchar(255),
    Salary int,
    PRIMARY KEY (ID)
);

Output

Query OK, 0 rows affected (0.21 sec)

Explanation

This SQL command creates a table named ‘Employees’ with five fields: ID, Name, Age, Address, Salary. The ID field is defined as the PRIMARY KEY. This means that every record in the table must have a unique ID.

Example

CREATE TABLE Students (
    Student_ID INT PRIMARY KEY,
    First_Name VARCHAR(40) NOT NULL,
    Last_Name VARCHAR(40) NOT NULL
);

Output

QUERY OK, 0 rows affected (0.01 sec)

Explanation

The above command creates a table Students with three columns - Student_ID, First_Name, and Last_Name. The Student_ID is specified as the PRIMARY KEY, which uniquely identifies each record in the table. No two records can have the same Student_ID and this column cannot contain NULL values.

Example

CREATE DATABASE TestDB;
GO

USE TestDB;
GO

CREATE TABLE Employees (
   ID INT PRIMARY KEY,
   Name NVARCHAR(50) NOT NULL,
);
GO

Output

Command(s) completed successfully.

Explanation

In the example, a new database, TestDB, is created, and a new table, Employees, is created within that database. The Employees table contains two columns, ID and Name. The ID column is designated as the primary key for the table. This is indicated by the PRIMARY KEY declaration following the ID INT statement. The primary key is a unique identifier for each record in the table.

Example

CREATE TABLE Customers (
    CustomerID int NOT NULL PRIMARY KEY,
    FirstName varchar(255),
    LastName varchar(255),
    City varchar(255)
);

Output

Table CUSTOMERS created.

Explanation

In the provided example, a new table named ‘Customers’ is created with four columns: CustomerID, FirstName, LastName, and City. Here, CustomerID is set as a PRIMARY KEY, which uniquely identifies each record in the database table.

Example

CREATE TABLE Students (
    ID INT PRIMARY KEY,
    NAME TEXT,
);
INSERT INTO Students(ID, NAME) VALUES (1, 'John');

Output

Query OK, 0 rows affected (0.01 sec)

Explanation

In the given example, the Students table is created with ID and NAME columns. The ID column is defined as the PRIMARY KEY. This implies that the ID column cannot hold duplicate or null values. After defining the table, a row containing the values (1, 'John') is inserted.