modify table add column constraint in sql code example

Example 1: sql add column

ALTER TABLE Customers
ADD Email varchar(255);

Example 2: sql alter table

-- With check if field already exists

IF NOT EXISTS(SELECT 1 FROM sys.columns 
          WHERE Name = N'Email'
          AND Object_ID = Object_ID(N'dbo.Customers'))
BEGIN    
	ALTER TABLE Customers
    ADD Email varchar(255);
END