From ff802c3f757c7b4b7f3c05598316616dbe11f9f0 Mon Sep 17 00:00:00 2001
From: GowthamRaj K <43011442+gowthamrajk@users.noreply.github.com>
Date: Tue, 27 Jul 2021 19:23:51 +0530
Subject: [PATCH] Update README.md

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 README.md | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

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@@ -154,3 +154,33 @@ These components are −
 - A field with a NULL value is the one that has been left blank during a record creation.
 
 # SQL Constraints 🚩
+
+- Constraints are the rules enforced on data columns on a table. 
+- These are used to limit the type of data that can go into a table. 
+- This ensures the accuracy and reliability of the data in the database.
+- Constraints can either be column level or table level. 
+- Column level constraints are applied only to one column whereas, table level constraints are applied to the entire table.
+
+Following are some of the most commonly used constraints available in SQL −
+
+### NOT NULL Constraint 
+− Ensures that a column cannot have a NULL value.
+
+### DEFAULT Constraint 
+− Provides a default value for a column when none is specified.
+
+### UNIQUE Constraint 
+− Ensures that all the values in a column are different.
+
+### PRIMARY Key 
+− Uniquely identifies each row/record in a database table.
+
+### FOREIGN Key 
+− Uniquely identifies a row/record in any another database table.
+
+### CHECK Constraint 
+− The CHECK constraint ensures that all values in a column satisfy certain conditions.
+
+### INDEX  
+− Used to create and retrieve data from the database very quickly.
+