Altering table adding columns

Alter Table TableName

Add    [ColumnName1] [float] NULL,
           [ColumnName2] [nvarchar](50) NULL,
          

Custom 404 Error Page

Write in .htaccess

ErrorDocument 404 /404-error-page.html

Change Where  404-error-page.html to your error page

Disallow folders using Robots.txt

User-agent: *
Disallow: /folder1/
Disallow: /folder2/

Where folder1 & folder2 are the folders which should be hidden from the search engines