Constraints in SQL

 Overview

In this article discuss about the Constraints in SQL. Why it is required for any developer before create the table or fully database for our projects or application. Why it's very import phase for any type of database define  or design for a our application.

 Constraints

It's define some condition when we perform any DML Operation(Data Manipulation Language Syntax like Insert new Record,Update New Record and also perform the Delete operation) in the database.

Suppose we have a master data table and we want to delete some rows of master table if any row or rows inserted childeren table behalf of this master table records if deletion is perform so childeren table data is waste or ambiguous data it's contains memory.

So how to prevent the this type of activity inside the database through Constraint.

below some constraint so one by one discuss.

 SQL Constraints

1. NOT NULL
2. Unique
3. Primary Key
4. Foreign Key
5. Check
6. Default

 Not Null Constraint

 If we create a table with NOT NULL  Constraints. So that mean in this particular column value we can not empty inserted because that's show error so please see in below picture.

First We create a table with Not Null Constraints:-

Picture for create table with NOTNULL Constraints

Now Try to insert with null value

Showing Error when we try to insert data with null values

 Unique Constraint

If we create or apply with any column unique constraint that's mean this column all row values is unique. Below images show the output.

Now create table with Unique constraints

Create table with Unique constraints

Now try to some data insert with unique constraints

Insert data with unique constraints

Show the OUTPUT below Picture

 Explaination

Here first two insertion query is different value with Email column but last two insertion query with same first two insertion query. so that's why below two row insertion failed. 

 Primary Key Constraint