SQL Foreign key Constraint

A Foreign key in one table points to a primary key in another table.

Firstly we create two tables:-

“employee” table: -

E_Id	Name	Address
1	Sachin	Kanpur
2	Amit	Aligarh
3	Sumt	Etah

“Emp” table

Emp_id	dept	E_Id
1	IT	2
2	sales	1

The SQL Foreign key constraint on create table

Create table Emp

(

Emp_id int NOT NULL PRIMARY KEY,

dept varchar (250) NOT NULL,

E_Id int Foreign Key references employee(E_Id)

)

SQL foreign Key constraint on ALTER Table

ALTER Table Emp

Add foreign key (E_Id)

References Employee (E_Id)

Drop a Foreign Key Constraint –

Alter table Emp

Drop contraint E_Id

                                                Author : - Sachin Pathak

SQL Primary Key constraints

The primary key is used to uniquely identify each record in the table. Primary key contain unique value .The primary key cannot contain NULL value, each table contain only one primary key.

Primary key on create table –

Create table employee

Id int NOT NULL,

First name varchar(250) NOT NULL,

Last name varchar(250),

City varchar(250),

Salary int,

CONSTRAINT emp_id PRIMARY KEY (ID, First Name)

Primary key on alter table-

ALTER table employee

Add primary key(Id)

Drop a Primary Key –

ALTER table employee

Drop CONSTRAINT emp_id

                                                             Author - Sachin Pathak

The SQL UNION Operator

The UNION operator is used to combine the result of two or more SELECT Statement. The UNION operator work when each select statement have same number of column, the column must have same data type and also the order of the columns in the each SELECT statement be the same.

Example -

Firstly we create two table names emp and employee

select id from emp

union

select id from employee

 Output - 

                                                            Author - Sachin Pathak