KEY Concept

 

Query ‘

CREATE TABLE employees

( employee_id INT PRIMARY KEY,

  last_name VARCHAR(50) NOT NULL,

  first_name VARCHAR(50) NOT NULL,

  );

Add primary key in existing coloum

ALTER TABLE student alter column roll int NOT NULL

ALTER TABLE student  ADD PRIMARY KEY (roll)

CREATE FOREGIN KEY

 

CREATE TABLE products

( product_id INT PRIMARY KEY,

  product_name VARCHAR(50) NOT NULL,

  category VARCHAR(25)

);

 

CREATE TABLE inventory

( inventory_id INT PRIMARY KEY,

  product_id INT NOT NULL,

  quantity INT,

  min_level INT,

  max_level INT,

  CONSTRAINT fk_inv_product_id

    FOREIGN KEY (product_id)

    REFERENCES products (product_id)  //PRODUCTS  PARENTS TABLE 

);

Forigen key  alter table 

ALTER TABLE inventory
ADD CONSTRAINT nrsolu
    FOREIGN KEY (product_id)
    REFERENCES products (product_id);

Create primary key 

CREATE TABLE employees
( employee_id INT PRIMARY KEY,
  last_name VARCHAR(50) NOT NULL,
  first_name VARCHAR(50) NOT NULL,
  salary MONEY
);

 

Primary using  constraint

CREATE TABLE employees

( employee_id INT,

  last_name VARCHAR(50) NOT NULL,

  first_name VARCHAR(50) NOT NULL,

  salary MONEY,

  CONSTRAINT employees_pk PRIMARY KEY (employee_id)

);

Primary key alter command

ALTER TABLE employees

ADD CONSTRAINT employees_pk PRIMARY KEY (employee_id);

Unique key

 

CREATE TABLE employees

( employee_id INT PRIMARY KEY,

  employee_number INT NOT NULL,

  last_name VARCHAR(50) NOT NULL,

  first_name VARCHAR(50),

  salary MONEY,

  CONSTRAINT employees_unique UNIQUE (employee_number)

);

Unique using alter table

ALTER TABLE employees

ADD CONSTRAINT employees_unique UNIQUE (employee_number);