Redirect non www to www url

It is very important from SEO point of view for a site to open from single URL. If we want that uor site should only open with www preceding the url we should just copy and paste the code below in our .htaccess file and you are done.For example if a site opens with 'abc.com' and also with 'www.abc.com' and you want that it should always open with 'www.abc.com' and if one tries to open with 'abc.com' it should redirect it to 'www.abc.com'.

RewriteEngine On
RewriteCond %{HTTPS} off
RewriteCond %{HTTP_HOST} !^www\.(.*)$ [NC]
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]

RewriteCond %{HTTPS} on
RewriteCond %{HTTP_HOST} !^www\.(.*)$ [NC]
RewriteRule ^(.*)$ https://www.%{HTTP_HOST}/$1 [R=301,L]